Add label to theme-switcher button. (#543)

Fixes 
- #537 

as suggested by @jamesprime by adding an [aria
label](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Attributes/aria-label)
to labelless button. Confirmed with macOS's "VoiceOver" screen reader...

![Screenshot 2025-06-03 at 13 25
10](https://github.com/user-attachments/assets/93c86456-feca-4ff4-91d4-4b8c4afd5797)

---

As with the ☼ / ☾ it's a bit WET because it needs declaring and then the
same lines appear in the switch conditional.

Author: samcunliffe

docs/_includes/header_custom.html

@@ -1,15 +1,19 @@
-<button class="btn js-toggle-dark-mode">☼</button>
+<button class="btn js-toggle-dark-mode" aria-label="Switch to light mode">
+  ☼
+</button>
 
 <script>
   const toggleDarkMode = document.querySelector(".js-toggle-dark-mode");
   jtd.addEvent(toggleDarkMode, "click", function () {
     if (jtd.getTheme() === "light") {
       jtd.setTheme("dark");
       toggleDarkMode.textContent = "☼";
+      toggleDarkMode.ariaLabel = "Switch to light mode";
       localStorage.setItem("theme", "dark");
     } else {
       jtd.setTheme("light");
       toggleDarkMode.textContent = "☾";
+      toggleDarkMode.ariaLabel = "Switch to dark mode";
       localStorage.setItem("theme", "light");
     }
   });