Dark mode support

Summary:
This change addresses some obvious issues which made the ui-debugger unusable with dark mode on.

There may more things that need changing, but at the very least this should be a good place to start.

Reviewed By: fabiomassimo

Differential Revision: D43083218

fbshipit-source-id: 8e4338b79178d3a0f05f9bcaffa2fc6f35eb0e21

desktop/plugins/public/ui-debugger/components/Tree.tsx

@@ -348,7 +348,7 @@ const TreeItemRow = styled.li<{
     ? 'rgba(255,0,0,.3)'
     : isSelected
     ? theme.selectionBackgroundColor
-    : theme.white,
+    : theme.backgroundDefault,
 }));
 
 function ExpandedIconOrSpace(props: {