apply light/dark theme immediately as ims.js is run

this prevents the flickering seen in the current approach, since
it waits until after the body is drawn to determine whether the
page should be light or dark

src/ims/element/static/ims.js

@@ -17,29 +17,25 @@
 //
 // Adapted from https://getbootstrap.com/docs/5.3/customize/color-modes/#javascript
 // Under Creative Commons Attribution 3.0 Unported License
+const getStoredTheme = () => localStorage.getItem("theme");
+const setStoredTheme = theme => localStorage.setItem("theme", theme);
+const getPreferredTheme = () => {
+    const storedTheme = getStoredTheme();
+    if (storedTheme) {
+        return storedTheme;
+    }
+    return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+};
+const setTheme = theme => {
+    if (theme === "auto") {
+        document.documentElement.setAttribute("data-bs-theme", (window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light"));
+    } else {
+        document.documentElement.setAttribute("data-bs-theme", theme);
+    }
+};
 function applyTheme() {
     "use strict";
 
-    const getStoredTheme = () => localStorage.getItem("theme");
-    const setStoredTheme = theme => localStorage.setItem("theme", theme);
-
-    const getPreferredTheme = () => {
-        const storedTheme = getStoredTheme();
-        if (storedTheme) {
-            return storedTheme;
-        }
-
-        return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
-    };
-
-    const setTheme = theme => {
-        if (theme === "auto") {
-            document.documentElement.setAttribute("data-bs-theme", (window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light"));
-        } else {
-            document.documentElement.setAttribute("data-bs-theme", theme);
-        }
-    };
-
     setTheme(getPreferredTheme());
 
     const showActiveTheme = (theme, focus = false) => {
@@ -88,6 +84,10 @@ function applyTheme() {
         });
     });
 }
+// Set the theme immediately, before the rest of the page loads. We need to come back later
+// to invoke applyTheme(), as that will only work once the navbar has been drawn (with its
+// dropdown theme selector.
+setTheme(getPreferredTheme());
 
 
 //