change dropdown colors to see easily

src/client/styles/scss/theme/christmas.scss

@@ -91,9 +91,9 @@ html[dark] {
   $border-color-theme: #ccc; // former: `$navbar-border: #ccc;`
 
   // Dropdown colors
-  $bgcolor-dropdown-link-active: $growi-blue;
-  $color-dropdown-link-active: $color-reversal;
-  $color-dropdown-link-hover: $color-global;
+  $bgcolor-dropdown-link-active: $primary;
+  $color-dropdown-link-active: $color-global;
+  $color-dropdown-link-hover: $color-reversal;
 
   // admin theme box
   $color-theme-color-box: lighten($themecolor, 20%);