Auto merge of #78773 - GuillaumeGomez:theme-picker-shortcut, r=jyn514

Add shortcut for theme picker menu

Follow-up of #78584

Just like you can focus the search input by pressing "S", you can now access the theme picker menu by pressing "T" and navigate through the options only using the keyboard.

cc `@notriddle`

r? `@jyn514`

src/librustdoc/html/static/main.js

@@ -415,6 +415,15 @@ function defocusSearchBar() {
                 displayHelp(true, ev);
                 break;
 
+            case "t":
+            case "T":
+                displayHelp(false, ev);
+                ev.preventDefault();
+                var themePicker = getThemePickerElement();
+                themePicker.click();
+                themePicker.focus();
+                break;
+
             default:
                 var themePicker = getThemePickerElement();
                 if (themePicker.parentNode.contains(ev.target)) {
@@ -2852,6 +2861,7 @@ function defocusSearchBar() {
         var shortcuts = [
             ["?", "Show this help dialog"],
             ["S", "Focus the search field"],
+            ["T", "Focus the theme picker menu"],
             ["↑", "Move up in search results"],
             ["↓", "Move down in search results"],
             ["↹", "Switch tab"],