Fix toggling dark mode with highlightjs: false (in mkdocs theme)

When highlightjs is disabled, some HTML elements don't exist. But the code didn't expect this and had an error, interrupting the entire action of toggling dark mode.

Author: oprypin

packages/properdocs-theme-mkdocs/properdocs_theme_mkdocs/js/darkmode.js

@@ -1,8 +1,11 @@
 function setColorMode(mode) {
     // Switch between light/dark theme. `mode` is a string value of either 'dark' or 'light'.
+    document.documentElement.setAttribute('data-bs-theme', mode);
     var hljs_light = document.getElementById('hljs-light'),
         hljs_dark = document.getElementById('hljs-dark');
-    document.documentElement.setAttribute('data-bs-theme', mode);
+    if (hljs_light == null || hljs_dark == null) {
+        return;
+    }
     if (mode == 'dark') {
         hljs_light.disabled = true;
         hljs_dark.disabled = false;