diff --git a/app/assets/css/app.css b/app/assets/css/app.css
index 43f652e..8b464fb 100644
--- a/app/assets/css/app.css
+++ b/app/assets/css/app.css
@@ -27,7 +27,11 @@
   --color-base-200: oklch(94% 0.012 75);
   --color-base-300: oklch(90% 0.016 75);
   --color-base-content: oklch(25% 0.015 50);
-  --color-primary: oklch(58% 0.16 45);
+  /* 54% lightness, not 58%: primary is also *text* (pagination links, body
+     links, the post-title hover). At 58% it measured 4.18:1 on base-100, below
+     the 4.5:1 floor; 54% is 4.93:1 there and 4.52:1 on base-200. Same hue and
+     chroma - this is a contrast fix, not a re-colour. */
+  --color-primary: oklch(54% 0.16 45);
   --color-primary-content: oklch(98% 0.01 75);
   --color-secondary: oklch(55% 0.12 30);
   --color-secondary-content: oklch(98% 0.008 30);
@@ -53,11 +57,18 @@
   --noise: 0;
 }
 
-/* Warm dark theme */
+/* Warm dark theme.
+   `prefersdark: false` deliberately: daisyUI emits an *unscoped*
+   `@media (prefers-color-scheme: dark) { :root { … } }` for a prefersdark
+   theme, which lands after the light block and beats `[data-theme="light"]` on
+   equal specificity - an explicit light choice rendered dark on a dark-OS
+   browser. The inline script in components/layouts/root.html.heex resolves
+   "system" to an explicit data-theme instead, so no rule here may follow the
+   OS preference on its own. */
 @plugin "../vendor/daisyui-theme" {
   name: "dark";
   default: false;
-  prefersdark: true;
+  prefersdark: false;
   color-scheme: "dark";
   --color-base-100: oklch(24% 0.015 50);
   --color-base-200: oklch(20% 0.012 50);
@@ -144,12 +155,12 @@ body { font-family: 'Source Sans 3', sans-serif; }
 }
 
 .blogex-post-preview header h2 a {
-  color: oklch(var(--color-base-content));
+  color: var(--color-base-content);
   text-decoration: none;
 }
 
 .blogex-post-preview header h2 a:hover {
-  color: oklch(var(--color-primary));
+  color: var(--color-primary);
 }
 
 .blogex-post-description {
@@ -163,7 +174,10 @@ body { font-family: 'Source Sans 3', sans-serif; }
   align-items: center;
   gap: 0.5rem;
   font-size: 0.875rem;
-  opacity: 0.6;
+  /* 0.6 measured 4.16:1 (base-content over base-100, light theme) - below the
+     4.5:1 floor. 0.625 is what the floor needs; 0.65 keeps a margin without
+     making the meta line as loud as the description (0.7 -> 5.66:1). */
+  opacity: 0.65;
   margin-top: 0.25rem;
 }
 
@@ -172,7 +186,7 @@ body { font-family: 'Source Sans 3', sans-serif; }
   padding: 0.125rem 0.5rem;
   border-radius: 9999px;
   font-size: 0.75rem;
-  background: oklch(var(--color-base-200));
+  background: var(--color-base-200);
 }
 
 .blogex-post-header h1 {
@@ -193,9 +207,9 @@ body { font-family: 'Source Sans 3', sans-serif; }
 .blogex-post-body p { margin-top: 1rem; }
 .blogex-post-body ul { list-style: disc; padding-left: 1.5rem; margin-top: 0.5rem; }
 .blogex-post-body ol { list-style: decimal; padding-left: 1.5rem; margin-top: 0.5rem; }
-.blogex-post-body pre { background: oklch(var(--color-base-200)); padding: 1rem; border-radius: 0.5rem; overflow-x: auto; margin-top: 1rem; }
+.blogex-post-body pre { background: var(--color-base-200); padding: 1rem; border-radius: 0.5rem; overflow-x: auto; margin-top: 1rem; }
 .blogex-post-body code { font-size: 0.875em; }
-.blogex-post-body a { color: oklch(var(--color-primary)); text-decoration: underline; }
+.blogex-post-body a { color: var(--color-primary); text-decoration: underline; }
 
 /* External link marker on links rewritten by Blogex.ExternalLinks (class
    `blogex-external`, added at compile time together with target="_blank").
@@ -235,7 +249,7 @@ body { font-family: 'Source Sans 3', sans-serif; }
   border-width: 0;
 }
 
-.blogex-post-body blockquote { border-left: 3px solid oklch(var(--color-primary)); padding-left: 1rem; margin-top: 1rem; margin-bottom: 1rem; font-style: italic; opacity: 0.85; }
+.blogex-post-body blockquote { border-left: 3px solid var(--color-primary); padding-left: 1rem; margin-top: 1rem; margin-bottom: 1rem; font-style: italic; opacity: 0.85; }
 
 .blogex-tag-list {
   display: flex;
@@ -248,13 +262,13 @@ body { font-family: 'Source Sans 3', sans-serif; }
   padding: 0.25rem 0.75rem;
   border-radius: 9999px;
   font-size: 0.875rem;
-  background: oklch(var(--color-base-200));
+  background: var(--color-base-200);
   text-decoration: none;
 }
 
 .blogex-tag-link:hover, .blogex-tag-active {
-  background: oklch(var(--color-primary));
-  color: oklch(var(--color-primary-content));
+  background: var(--color-primary);
+  color: var(--color-primary-content);
 }
 
 .blogex-pagination {
@@ -267,7 +281,7 @@ body { font-family: 'Source Sans 3', sans-serif; }
 }
 
 .blogex-pagination a {
-  color: oklch(var(--color-primary));
+  color: var(--color-primary);
   text-decoration: none;
 }
 
diff --git a/app/lib/firehose_web/components/layouts.ex b/app/lib/firehose_web/components/layouts.ex
index 1b0a1f2..b0bc720 100644
--- a/app/lib/firehose_web/components/layouts.ex
+++ b/app/lib/firehose_web/components/layouts.ex
@@ -84,7 +84,10 @@ defmodule FirehoseWeb.Layouts do
   def theme_toggle(assigns) do
     ~H"""
     <div class="card relative flex flex-row items-center border-2 border-base-300 bg-base-300 rounded-full">
-      <div class="absolute w-1/3 h-full rounded-full border-1 border-base-200 bg-base-100 brightness-200 left-0 [[data-theme=light]_&]:left-1/3 [[data-theme=dark]_&]:left-2/3 transition-[left]" />
+      <%!-- The knob follows the visitor's choice, not the resolved palette: in
+      system mode data-theme may be light or dark, but the system icon stays
+      selected. --%>
+      <div class="absolute w-1/3 h-full rounded-full border-1 border-base-200 bg-base-100 brightness-200 left-0 [[data-theme-choice=light]_&]:left-1/3 [[data-theme-choice=dark]_&]:left-2/3 transition-[left]" />
 
       <button
         class="flex p-2 cursor-pointer w-1/3"
diff --git a/app/lib/firehose_web/components/layouts/root.html.heex b/app/lib/firehose_web/components/layouts/root.html.heex
index 74b7879..7e09cf2 100644
--- a/app/lib/firehose_web/components/layouts/root.html.heex
+++ b/app/lib/firehose_web/components/layouts/root.html.heex
@@ -14,20 +14,37 @@
     </script>
     <script>
       (() => {
-        const setTheme = (theme) => {
-          if (theme === "system") {
-            localStorage.removeItem("phx:theme");
-            document.documentElement.removeAttribute("data-theme");
-          } else {
-            localStorage.setItem("phx:theme", theme);
-            document.documentElement.setAttribute("data-theme", theme);
-          }
+        // Theme selection is always explicit: the document always carries a
+        // data-theme attribute, resolved here before first paint. An explicit
+        // choice therefore always wins in CSS, whatever the OS preference is;
+        // no stylesheet rule follows the OS on its own.
+        //
+        // data-theme-choice records what the visitor actually chose (unset,
+        // system, light, dark) and is what the toggle's knob follows: in
+        // "system" mode the palette may resolve to light or dark, but the
+        // system icon must stay selected.
+        const prefersDark = window.matchMedia("(prefers-color-scheme: dark)");
+        const stored = () => localStorage.getItem("phx:theme");
+        const resolve = (theme) =>
+          theme === "system" ? (prefersDark.matches ? "dark" : "light") : theme;
+        const apply = (theme) => {
+          document.documentElement.setAttribute("data-theme", resolve(theme));
+          document.documentElement.setAttribute("data-theme-choice", theme);
         };
+        const setTheme = (theme) => {
+          localStorage.setItem("phx:theme", theme);
+          apply(theme);
+        };
+        // Light is the default: no stored preference means light, not "follow
+        // the OS". "System" is an explicit choice and is stored as such.
         if (!document.documentElement.hasAttribute("data-theme")) {
-          setTheme(localStorage.getItem("phx:theme") || "system");
+          apply(stored() || "light");
         }
-        window.addEventListener("storage", (e) => e.key === "phx:theme" && setTheme(e.newValue || "system"));
-        
+        // Follow the OS only while the visitor's recorded choice is "system".
+        prefersDark.addEventListener("change", () => {
+          if (stored() === "system") apply("system");
+        });
+        window.addEventListener("storage", (e) => e.key === "phx:theme" && setTheme(e.newValue || "light"));
         window.addEventListener("phx:set-theme", (e) => setTheme(e.target.dataset.phxTheme));
       })();
     </script>
diff --git a/app/test/firehose_web/theme_stylesheet_test.exs b/app/test/firehose_web/theme_stylesheet_test.exs
new file mode 100644
index 0000000..8135266
--- /dev/null
+++ b/app/test/firehose_web/theme_stylesheet_test.exs
@@ -0,0 +1,188 @@
+defmodule FirehoseWeb.ThemeStylesheetTest do
+  @moduledoc """
+  Guards the theme's source files against the two defects the theme yak records.
+
+  daisyUI 5 stores a *complete* colour in every `--color-*` custom property
+  (`--color-base-content: oklch(25% 0.015 50)`), so `oklch(var(--color-…))`
+  re-wraps a complete value and the browser drops the whole declaration. The
+  visible symptom was a transparent chip background and unstyled links.
+
+  Each theme's `prefersdark` option makes daisyUI emit an *unscoped*
+  `@media (prefers-color-scheme: dark) { :root { … } }`, which lands after the
+  light block and wins on equal specificity: an explicit `data-theme="light"`
+  rendered dark on a dark-OS browser. Theme selection now lives in the inline
+  script in `root.html.heex`, which always writes an explicit `data-theme`
+  before first paint, so no stylesheet rule may follow the OS on its own.
+
+  These read the two source files as text: they are regression guards, not
+  behavioural tests. The authoritative check for *rendered* colour and theme
+  selection is the factory probe (`mastra-factory/scripts/theme-probe.mjs`),
+  which needs a browser; `make verify` never parses or renders CSS.
+  """
+  use ExUnit.Case, async: true
+
+  # `__DIR__`-relative so the assertions do not depend on the working directory.
+  @stylesheet Path.expand("../../assets/css/app.css", __DIR__)
+  @root_layout Path.expand("../../lib/firehose_web/components/layouts/root.html.heex", __DIR__)
+  @layouts Path.expand("../../lib/firehose_web/components/layouts.ex", __DIR__)
+
+  # Measured by the factory probe (chromium, 2026-10-29) for `.blogex-post-meta`
+  # in the light theme: base-content over base-100 composited at 0.6 opacity is
+  # 4.16:1, below the 4.5:1 acceptance floor. The alpha the floor needs is
+  # 0.625; anything below that fails.
+  @meta_min_opacity 0.625
+
+  # Same probe, same day, for `--color-primary` as *text* on `--color-base-100`
+  # in the light theme - what pagination links, body links and the post-title
+  # hover now use. 58% lightness measured 4.18:1; 56% is 4.54:1, too thin a
+  # margin; 54% is 4.93:1 (and 4.52:1 on base-200, the lightest chip ground).
+  @light_primary_max_lightness 0.54
+
+  describe "colour tokens" do
+    test "theme colour tokens are used as complete values" do
+      offenders =
+        @stylesheet
+        |> read()
+        |> String.split("\n")
+        |> Enum.with_index(1)
+        |> Enum.filter(fn {line, _number} -> line =~ ~r/oklch\(\s*var\(--color-/ end)
+
+      assert offenders == [],
+             """
+             oklch(var(--color-…)) is invalid: the --color-* properties already hold a
+             complete oklch(…) value, so the declaration is dropped. Use var(--color-…)
+             directly, or color-mix() where a derived colour is genuinely needed.
+
+             Offending line(s):
+             #{Enum.map_join(offenders, "\n", fn {line, number} -> "  #{number}: #{String.trim(line)}" end)}
+             """
+    end
+  end
+
+  describe "theme selection" do
+    test "the head script always writes an explicit data-theme" do
+      script = theme_script()
+
+      # "system" is resolved here, from the OS preference, before first paint.
+      assert script =~ ~r/matchMedia\(\s*["']\(prefers-color-scheme:\s*dark\)["']\s*\)/
+      assert script =~ ~r/setAttribute\(\s*["']data-theme["']/
+      # Removing the attribute is what let the unstyled root fall back to the
+      # OS-driven media block, so "system" must never be expressed that way.
+      refute script =~ "removeAttribute"
+    end
+
+    test "an OS preference change re-resolves only while 'system' is the choice" do
+      assert theme_script() =~
+               ~r/addEventListener\(\s*["']change["'][\s\S]*?stored\(\)\s*===\s*["']system["']/
+    end
+
+    test "the toggle knob follows the choice, not the resolved palette" do
+      # A system choice can resolve to the dark palette; the indicator must still
+      # sit on the system icon, or the toggle lies about what is selected.
+      layouts = read(@layouts)
+
+      assert layouts =~ "[[data-theme-choice=light]_&]:left-1/3"
+      assert layouts =~ "[[data-theme-choice=dark]_&]:left-2/3"
+      refute layouts =~ "[[data-theme=light]_&]"
+      refute layouts =~ "[[data-theme=dark]_&]"
+    end
+
+    # The human's decision (REVIEW.md, Q1): "default to light theme, and make
+    # setting explicit". Light is what an unset preference resolves to; "system"
+    # is an explicit choice, not the default.
+    test "no stored preference means the light theme" do
+      assert theme_script() =~ ~r/apply\(\s*stored\(\)\s*\|\|\s*["']light["']\s*\)/
+    end
+
+    test "no theme follows the OS preference on its own" do
+      # `prefersdark: true` mints the unscoped media block that outranked the
+      # explicit attribute. Selection is the script's job; the stylesheet's
+      # default (light) must apply whenever an attribute is absent.
+      assert theme_option("light", "prefersdark") == "false"
+      assert theme_option("dark", "prefersdark") == "false"
+    end
+  end
+
+  describe "post meta contrast" do
+    test "the post meta is dimmed no further than the contrast floor allows" do
+      opacity = declaration(".blogex-post-meta", "opacity")
+
+      assert opacity >= @meta_min_opacity,
+             """
+             .blogex-post-meta has opacity #{opacity}, which composites base-content over
+             base-100 below the 4.5:1 floor (probe: 0.6 -> 4.16:1, 0.625 -> 4.51:1).
+             Raise it, or the meta line is unreadable in the light theme.
+             """
+    end
+  end
+
+  describe "light theme link colour" do
+    test "primary is dark enough to be text on base-100" do
+      lightness = lightness(theme_option("light", "--color-primary"))
+
+      assert lightness <= @light_primary_max_lightness,
+             """
+             The light theme's --color-primary has #{lightness} lightness, so link text
+             (pagination, body links, the post-title hover) composites below 4.5:1 on
+             base-100: the probe measured 4.18:1 at 0.58. 0.54 measures 4.93:1.
+             """
+    end
+  end
+
+  defp read(path), do: File.read!(path)
+
+  # OKLCH lightness as a 0..1 fraction; daisyUI writes either `54%` or `0.54`.
+  defp lightness(value) do
+    case Regex.run(~r/^oklch\(\s*([\d.]+)\s*(%?)/, value) do
+      [_, number, "%"] -> number |> Float.parse() |> elem(0) |> Kernel./(100)
+      [_, number, ""] -> number |> Float.parse() |> elem(0)
+      _ -> flunk("cannot read the lightness of #{inspect(value)}")
+    end
+  end
+
+  # The declarations of a single plain rule (`selector { … }`).
+  defp declaration(selector, property) do
+    selector
+    |> rule_body()
+    |> then(fn body ->
+      case Regex.run(~r/^\s*#{property}:\s*([^;]+);/m, body) do
+        [_, value] -> value |> String.trim() |> String.to_float()
+        nil -> flunk("no #{property} in #{selector}")
+      end
+    end)
+  end
+
+  defp rule_body(selector) do
+    case Regex.run(~r/#{Regex.escape(selector)}\s*\{([^}]*)\}/, read(@stylesheet)) do
+      [_, body] -> body
+      nil -> flunk("no rule for #{selector} in #{@stylesheet}")
+    end
+  end
+
+  # The inline script in <head>, not the deferred `app.js` tag.
+  defp theme_script do
+    @root_layout
+    |> read()
+    |> then(&Regex.scan(~r/<script[^>]*>(.*?)<\/script>/s, &1, capture: :all_but_first))
+    |> List.flatten()
+    |> Enum.find(fn body -> body =~ "data-theme" end) ||
+      flunk("no inline theme script in #{@root_layout}")
+  end
+
+  # The body of the `@plugin "../vendor/daisyui-theme" { … }` block for `name`.
+  defp theme_block(name) do
+    @stylesheet
+    |> read()
+    |> String.split(~s(@plugin "../vendor/daisyui-theme" {))
+    |> Enum.find(fn block ->
+      Enum.any?(String.split(block, "\n"), &(&1 |> String.trim() == ~s(name: "#{name}";)))
+    end) || flunk("no daisyui-theme block named #{inspect(name)} in #{@stylesheet}")
+  end
+
+  defp theme_option(name, option) do
+    case Regex.run(~r/^\s*#{option}:\s*([^;]+);/m, theme_block(name)) do
+      [_, value] -> String.trim(value)
+      nil -> nil
+    end
+  end
+end