Fix error linting popup styles in light theme

src/rstudio/_rstudio-light.scss

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