Add tooltip to theme selector

docs/assets/plugins/theme/theme.js

@@ -10,6 +10,10 @@
       const noTransitions = Object.assign(document.createElement('style'), {
         textContent: '* { transition: none !important; }'
       });
+      const tooltip = Object.assign(document.createElement('sl-tooltip'), {
+        content: 'Toggle dark mode',
+        placement: 'left'
+      });
       const toggle = Object.assign(document.createElement('sl-icon-button'), {
         name: isDark ? 'sun' : 'moon',
         label: 'Toggle dark mode'
@@ -35,7 +39,8 @@
         });
       });
 
-      sidebarToggle.insertAdjacentElement('afterend', toggle);
+      tooltip.appendChild(toggle);
+      sidebarToggle.insertAdjacentElement('afterend', tooltip);
     });
   });
 })();