Fix dark theme lint error popup

src/rstudio/_rstudio-dark.scss

     }
   }
   .gwt-PopupPanel .popupContent {
-  	background: $ui-completions-color;
+  	background: $ui-completions-background;
+  	padding: 2px;
   	table {
       background: $ui-completions-color;
       border: 0;