apply gray-50 to only the light mode

src/client/styles/scss/theme/_apply-colors-light.scss

@@ -185,6 +185,8 @@ $border-color: $border-color-global;
  * GROWI on-edit
  */
 .grw-editor-navbar-bottom {
+  background-color: $gray-50;
+
   #slack-mark-white {
     display: none;
   }

src/client/styles/scss/theme/_apply-colors.scss

@@ -433,10 +433,6 @@ body.on-edit {
       border-top-color: $border-color-theme;
     }
   }
-
-  .grw-editor-navbar-bottom {
-    background-color: $gray-50;
-  }
 }
 
 /*