MarkerTimeline hover background color

Summary:
The hover background colour had complete disregard on the selected theme, both light and dark.

This change removes the gradient and instead sets a right hover background.

Reviewed By: antonk52

Differential Revision: D43872161

fbshipit-source-id: 4f0bfd97a2cb6bb9ceb7869dd38b37dc68ad6bb7

desktop/flipper-plugin/src/ui/MarkerTimeline.tsx

@@ -66,7 +66,7 @@ const Point = styled(Layout.Horizontal)<{
   alignItems: 'flex-start',
   lineHeight: '16px',
   ':hover': {
-    background: `linear-gradient(to top, ${theme.black} 0, ${theme.white} 10px)`,
+    background: theme.selectionBackgroundColor,
     paddingBottom: 5,
     zIndex: 2,
     '> span': {