After GitHub update, some selectors changed and the extension did not work properly.

Fixes:
file tree did not appear if there was a file with no extension, it has been fixed now
the selector of the repo details in the "Files changed" panel has been updated so it becomes hidden again on this panel to let us focus on the details
the send review form overlay selector fixed, so Holy approve and Needs work buttons are working again

Additions:
the file headers background colour has been changed to a light yellow to become more prominent, so when not checking a specific file, but rather scrolling through the changed files, it is easier to catch that a new file is started.