Given that your screen shot is showing the little down arrow next to Inbox, that suggests that your mouse is hovering over the Inbox label field. When I do that using the "light" theme, I get the same grey highlighting. If I move the mouse away then the highlighting goes away too as does the down arrow.
Just viewing your inbox with the mouse hovering nowhere near the inbox label shouldn't produce either the highlighting or the down arrow. So if you're getting that without the mouse being near there, then I can't explain it. Unless the same effect can be achieved by tabbing around the screen until inbox is selected.
Did you think of changing your Gmail display theme? There are plenty available via the options cog-wheel just below Settings.