diff --git a/astro/astro.config.mjs b/astro/astro.config.mjs
index 6cdfd8d6..f5a30182 100644
--- a/astro/astro.config.mjs
+++ b/astro/astro.config.mjs
@@ -66,6 +66,8 @@ export default defineConfig({
           'facebook', 'instagram', 'linkedin', 'rss-fill', 'tiktok', 'globe', 'mastodon', 'twitter',
           // Descriptive
           'gift-fill', 'pencil-fill', 'people-fill', 'person-fill', 'puzzle-fill', 'stopwatch-fill', 'tools',
+          // Theme toggle
+          'sun-fill', 'moon-fill', 'circle-half', 'check-lg',
         ],
         // CoreUI Brands
         cib: [
diff --git a/astro/src/components/Header.astro b/astro/src/components/Header.astro
index 1b8b3009..fcff7449 100644
--- a/astro/src/components/Header.astro
+++ b/astro/src/components/Header.astro
@@ -77,7 +77,7 @@ const breadCrumbs: Breadcrumbs =
 </header>
 <slot />
 
-<style lang="scss">
+<style is:global lang="scss">
   /* first declaration that is commented out below is for debugging purposes. */
   // * {
   //    outline: 1px solid white;
diff --git a/astro/src/components/ThemeToggle.astro b/astro/src/components/ThemeToggle.astro
new file mode 100644
index 00000000..6aedcc04
--- /dev/null
+++ b/astro/src/components/ThemeToggle.astro
@@ -0,0 +1,249 @@
+---
+import { Icon } from "astro-icon/components";
+
+interface Props {
+  class?: string;
+}
+
+const { class: classes = "" } = Astro.props;
+---
+
+<div class:list={["theme-toggle dropdown", classes]}>
+  <button
+    type="button"
+    id="theme-toggle"
+    class="theme-toggle-btn btn btn-outline-primary display-65 dropdown-toggle"
+    data-bs-toggle="dropdown"
+    data-theme="system"
+    aria-expanded="false"
+    aria-label="Color theme: Auto. Activate to change."
+  >
+    <span class="theme-toggle-icon" data-theme-icon="light" aria-hidden="true">
+      <Icon name="bi:sun-fill" class="bi" role="img" />
+    </span>
+    <span class="theme-toggle-icon" data-theme-icon="dark" aria-hidden="true">
+      <Icon name="bi:moon-fill" class="bi" role="img" />
+    </span>
+    <span class="theme-toggle-icon" data-theme-icon="system" aria-hidden="true">
+      <Icon name="bi:circle-half" class="bi" role="img" />
+    </span>
+    <span data-theme-label>Auto</span>
+  </button>
+  <ul class="dropdown-menu dropdown-menu-end px-15 pb-1" aria-labelledby="theme-toggle">
+    <li>
+      <button
+        type="button"
+        class="dropdown-item theme-option"
+        data-theme-option="light"
+        aria-pressed="false"
+      >
+        <Icon name="bi:sun-fill" class="bi me-2" role="img" aria-hidden="true" />
+        <span class="theme-option-label">Light</span>
+        <Icon name="bi:check-lg" class="bi ms-2 theme-option-check" role="img" aria-hidden="true" />
+      </button>
+    </li>
+    <li>
+      <button
+        type="button"
+        class="dropdown-item theme-option"
+        data-theme-option="dark"
+        aria-pressed="false"
+      >
+        <Icon name="bi:moon-fill" class="bi me-2" role="img" aria-hidden="true" />
+        <span class="theme-option-label">Dark</span>
+        <Icon name="bi:check-lg" class="bi ms-2 theme-option-check" role="img" aria-hidden="true" />
+      </button>
+    </li>
+    <li>
+      <button
+        type="button"
+        class="dropdown-item theme-option"
+        data-theme-option="system"
+        aria-pressed="false"
+      >
+        <Icon name="bi:circle-half" class="bi me-2" role="img" aria-hidden="true" />
+        <span class="theme-option-label">Auto</span>
+        <Icon name="bi:check-lg" class="bi ms-2 theme-option-check" role="img" aria-hidden="true" />
+      </button>
+    </li>
+  </ul>
+</div>
+
+<style is:global lang="scss">
+  @import "../styles/dark-mode.scss";
+
+  .theme-toggle .theme-toggle-btn {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    min-width: 7em;
+  }
+
+  /* Spacing between the active icon and the text label (when shown). */
+  .theme-toggle [data-theme-label] {
+    margin-inline-start: 0.25rem;
+  }
+
+  /* theme-icon=icon (default): show only the icon, drop the fixed width. */
+  .theme-toggle.theme-toggle--icon-only .theme-toggle-btn {
+    min-width: 0;
+  }
+  .theme-toggle.theme-toggle--icon-only [data-theme-label] {
+    display: none;
+  }
+
+  /* theme-loc placement spacing (left of / right of the Donate button). */
+  .theme-toggle.theme-toggle--left {
+    margin-inline: 0.25rem 1.5rem;
+  }
+  .theme-toggle.theme-toggle--right {
+    margin-inline: 0.5rem 0.25rem;
+  }
+
+  .theme-toggle .theme-toggle-icon {
+    display: none;
+  }
+  .theme-toggle .theme-toggle-btn[data-theme="light"] [data-theme-icon="light"],
+  .theme-toggle .theme-toggle-btn[data-theme="dark"] [data-theme-icon="dark"],
+  .theme-toggle .theme-toggle-btn[data-theme="system"] [data-theme-icon="system"] {
+    display: inline-flex;
+    align-items: center;
+  }
+
+  .theme-toggle .theme-option {
+    display: flex;
+    align-items: center;
+  }
+
+  .theme-toggle .theme-option-label {
+    flex: 1 1 auto;
+  }
+
+  .theme-toggle .theme-option-check {
+    visibility: hidden;
+  }
+  .theme-toggle .theme-option[aria-pressed="true"] .theme-option-check {
+    visibility: visible;
+  }
+
+  @include color-mode(dark) {
+    .theme-toggle .theme-toggle-btn.btn-outline-primary {
+      --bs-btn-color: var(--bs-white);
+      --bs-btn-border-color: var(--bs-white);
+      --bs-btn-hover-color: var(--bs-primary);
+      --bs-btn-hover-bg: var(--bs-white);
+      --bs-btn-hover-border-color: var(--bs-white);
+      --bs-btn-active-color: var(--bs-primary);
+      --bs-btn-active-bg: var(--bs-white);
+      --bs-btn-active-border-color: var(--bs-white);
+    }
+  }
+</style>
+
+<script>
+  type Mode = "light" | "dark" | "system";
+  const modes: Mode[] = ["light", "dark", "system"];
+  const labels: Record<Mode, string> = {
+    light: "Light",
+    dark: "Dark",
+    system: "Auto",
+  };
+
+  const retrieveTheme = (): Mode => {
+    try {
+      const stored = localStorage.getItem("theme");
+      return stored === "light" || stored === "dark" ? stored : "system";
+    } catch {
+      return "system";
+    }
+  };
+
+  const storeTheme = (mode: Mode) => {
+    try {
+      if (mode === "system") {
+        localStorage.removeItem("theme");
+      } else {
+        localStorage.setItem("theme", mode);
+      }
+    } catch {
+      // localStorage blocked — theme applies for this session but won't persist.
+    }
+  };
+
+  const init = () => {
+    const root = document.querySelector(".theme-toggle") as HTMLElement | null;
+    const button = document.getElementById(
+      "theme-toggle",
+    ) as HTMLButtonElement | null;
+    if (!root || !button) return;
+
+    // Layout is configurable via query params, resolved here because this is a
+    // static build with no per-request server. The button keeps a dynamic
+    // aria-label, so the icon-only mode is still announced to screen readers.
+    //   theme-loc=left|right   (default right: to the right of the Donate button)
+    //   theme-icon=icon|text   (default icon: icon-only, no visible text label)
+    const params = new URLSearchParams(window.location.search);
+    const placeLeft = params.get("theme-loc") === "left";
+    const showText = params.get("theme-icon") === "text";
+
+    root.classList.toggle("theme-toggle--icon-only", !showText);
+    root.classList.toggle("theme-toggle--left", placeLeft);
+    root.classList.toggle("theme-toggle--right", !placeLeft);
+
+    if (placeLeft) {
+      // Source order is Donate then toggle; move the toggle ahead of the
+      // mobile line break that precedes Donate so it sits to Donate's left.
+      const anchor =
+        root.parentElement?.querySelector(".w-100.d-lg-none") ??
+        root.parentElement?.querySelector('a.btn[href="/donate"]');
+      anchor?.before(root);
+    }
+
+    const applyTheme = (mode: Mode) => {
+      let themeToApply = mode;
+      if (themeToApply === "system") {
+        themeToApply = window.matchMedia("(prefers-color-scheme: dark)").matches
+          ? "dark"
+          : "light";
+      }
+      document.documentElement.setAttribute("data-bs-theme", themeToApply);
+    };
+
+    const updateButton = (mode: Mode) => {
+      const label = button.querySelector(
+        "[data-theme-label]",
+      ) as HTMLElement | null;
+      button.setAttribute("data-theme", mode);
+      if (label) label.textContent = labels[mode];
+      button.setAttribute(
+        "aria-label",
+        `Color theme: ${labels[mode]}. Activate to change.`,
+      );
+      root.querySelectorAll<HTMLButtonElement>(".theme-option").forEach((el) => {
+        const isCurrent = el.dataset.themeOption === mode;
+        el.setAttribute("aria-pressed", isCurrent ? "true" : "false");
+      });
+    };
+
+    const apply = (mode: Mode) => {
+      storeTheme(mode);
+      applyTheme(mode);
+      updateButton(mode);
+    };
+
+    apply(retrieveTheme());
+
+    root.querySelectorAll<HTMLButtonElement>(".theme-option").forEach((el) => {
+      el.addEventListener("click", () => {
+        const next = el.dataset.themeOption as Mode | undefined;
+        if (next && modes.includes(next)) apply(next);
+      });
+    });
+  };
+
+  if (document.readyState === "loading") {
+    window.addEventListener("DOMContentLoaded", init);
+  } else {
+    init();
+  }
+</script>
diff --git a/astro/src/components/navigation/NavSiteList.astro b/astro/src/components/navigation/NavSiteList.astro
index 72b2f94b..32617c9a 100644
--- a/astro/src/components/navigation/NavSiteList.astro
+++ b/astro/src/components/navigation/NavSiteList.astro
@@ -1,5 +1,6 @@
 ---
 import NavDropdownMenu from "./NavDropdownMenu.astro";
+import ThemeToggle from "../ThemeToggle.astro";
 
 interface Props {
   main?: boolean;
@@ -42,13 +43,20 @@ const { main = false, class: classes = "" } = Astro.props;
   </ul>
   {
     main && (
-      <a
-        class="btn btn-info btn-contrast-dark display-65 mx-1 my-1"
-        href="/donate"
-        role="button"
-      >
-        Donate
-      </a>
+      <Fragment>
+        <div class="w-100 d-lg-none" aria-hidden="true"></div>
+        <a
+          class="btn btn-info btn-contrast-dark display-65 mx-1 my-1"
+          href="/donate"
+          role="button"
+        >
+          Donate
+        </a>
+        {/* Default: icon-only, to the right of Donate. Query params override
+            this client-side (static build): ?theme-loc=left|right and
+            ?theme-icon=icon|text. See ThemeToggle.astro. */}
+        <ThemeToggle class="my-1 theme-toggle--icon-only theme-toggle--right" />
+      </Fragment>
     )
   }
 </Fragment>
diff --git a/astro/src/layouts/Layout.astro b/astro/src/layouts/Layout.astro
index 845e20a0..6fcc5a44 100644
--- a/astro/src/layouts/Layout.astro
+++ b/astro/src/layouts/Layout.astro
@@ -21,6 +21,39 @@ const { title, brandedTitle = false, crumbs, heading, metadata } = Astro.props;
 <!doctype html>
 <html lang="en">
   <head>
+    <script is:inline>
+      // Set data-bs-theme before paint to avoid a flash of the wrong theme.
+      // Reads localStorage; falls back to OS preference when no stored choice
+      // or when localStorage is blocked.
+      (function () {
+        function getStoredTheme() {
+          try {
+            var stored = localStorage.getItem("theme");
+            return stored === "light" || stored === "dark" ? stored : "system";
+          } catch (_) {
+            return "system";
+          }
+        }
+
+        function applyTheme() {
+          var theme = getStoredTheme();
+          if (theme === "system") {
+            theme = window.matchMedia("(prefers-color-scheme: dark)").matches
+              ? "dark"
+              : "light";
+          }
+          document.documentElement.setAttribute("data-bs-theme", theme);
+        }
+
+        applyTheme();
+
+        // While in "system" mode (no stored choice), follow live OS changes.
+        window
+          .matchMedia("(prefers-color-scheme: dark)")
+          .addEventListener("change", applyTheme);
+      })();
+    </script>
+
     <script>
       import { init } from '@plausible-analytics/tracker';
       init({
diff --git a/astro/src/styles/colors-and-themes.scss b/astro/src/styles/colors-and-themes.scss
index da710f61..8447acea 100644
--- a/astro/src/styles/colors-and-themes.scss
+++ b/astro/src/styles/colors-and-themes.scss
@@ -1,6 +1,6 @@
 @charset "UTF-8";
 
-$color-mode-type:                   media-query;
+$color-mode-type:                   data-attribute;
 $ac-navy:                           #041058;
 
 //*******************
diff --git a/astro/src/styles/dark-mode.scss b/astro/src/styles/dark-mode.scss
index c8d33188..f107964b 100644
--- a/astro/src/styles/dark-mode.scss
+++ b/astro/src/styles/dark-mode.scss
@@ -1,4 +1,5 @@
 // To be included in components and pages that need additional dark mode rules.
-$color-mode-type:                   media-query;
+$color-mode-type:                   data-attribute;
+
+@import "bootstrap/scss/mixins/_color-mode.scss";
 
-@import "bootstrap/scss/mixins/_color-mode.scss";
\ No newline at end of file