Fix dark theme support

Reviewed By: lblasa

Differential Revision: D50406262

fbshipit-source-id: eda5c73efcd50dbe572cab078d3c04a4cd845601

desktop/flipper-plugin/src/ui/PowerSearch/PowerSearchContainer.tsx

@@ -13,7 +13,7 @@ import {theme} from '../theme';
 
 const containerStyle = css`
   flex: 1 0 auto;
-  background-color: ${theme.white};
+  background-color: ${theme.backgroundDefault};
   display: flex;
   flex-direction: row;
   border-radius: ${theme.borderRadius};