handle auto theme correctly with the manual toggle

If the user used the manual toggle, they will not be able to get back to
auto since it will force set to light theme. This should fix that.

assets/js/themes.js

@@ -22,9 +22,11 @@ function setTheme(theme) {
     if (theme === THEME_DARK) {
         toggle_theme.children[0].className = 'icon ion-ios-sunny';
         document.body.className = 'dark-theme';
-    } else {
+    } else if (theme === THEME_LIGHT) {
         toggle_theme.children[0].className = 'icon ion-ios-moon';
         document.body.className = 'light-theme';
+    } else {
+        document.body.className = 'no-theme';
     }
 }