Fix dark mode issue

Summary: Fixed minor dark mode issue. Will address the ugly scrollbars in dark mode later.

Reviewed By: nikoant

Differential Revision: D28039447

fbshipit-source-id: eb0f545d224decb0334dca68b11277ddab612c2e

desktop/app/src/PluginContainer.tsx

@@ -65,7 +65,7 @@ const Container = styled(FlexColumn)({
 });
 
 export const SidebarContainer = styled(FlexRow)({
-  backgroundColor: colors.light02,
+  backgroundColor: theme.backgroundWash,
   height: '100%',
   overflow: 'auto',
 });