Persist light theme on refresh (#513)

Fixes 

- #414

... using the workaround suggested in 

-
just-the-docs/just-the-docs#1223 (comment).


Worth spending a bit of thought on whether we want more lines of my
hacky JS code saving the theme to local storage, which we potentially
can't delete until
[jtd/#1223](just-the-docs/just-the-docs#1223)
is solved nicely upstream.

### Question:

Is it worth switching build system to something that supports theme
toggling and inferring theme from the browser/os settings? It feels like
it's the responsibility of the doc engine/doc theme. E.g. [Material for
MkDocs supports
it](https://github.com/UCL-ARC/mkdocs-rc-docs/blob/140e8fe5eef0379997af1b2eaaf396786bc006b8/mkdocs-project-dir/mkdocs.yml#L28).

That said, if you're happy with these lines, I am.

Author: samcunliffe

docs/_includes/header_custom.html

@@ -2,11 +2,25 @@
 
 <script>
   const toggleDarkMode = document.querySelector(".js-toggle-dark-mode");
-
   jtd.addEvent(toggleDarkMode, "click", function () {
     if (jtd.getTheme() === "light") {
       jtd.setTheme("dark");
       toggleDarkMode.textContent = "☼";
+      localStorage.setItem("theme", "dark");
+    } else {
+      jtd.setTheme("light");
+      toggleDarkMode.textContent = "☾";
+      localStorage.setItem("theme", "light");
+    }
+  });
+
+  /* TODO: we can probably delete this (hacky workaround) if/when just-the-docs/#1223 is fixed.
+   * Meanwhile, we check each time if there is a theme written to local storage and obey it.
+   */
+  window.addEventListener("DOMContentLoaded", function () {
+    if (localStorage.getItem("theme") === "dark") {
+      jtd.setTheme("dark");
+      toggleDarkMode.textContent = "☼";
     } else {
       jtd.setTheme("light");
       toggleDarkMode.textContent = "☾";