Follow-up from "WIP: feature/#65 Implementation of multilingualism": backgrounds when hovering

The following discussion from !152 (merged) should be addressed:

• @mrodzis started a discussion: (+1 comment)

  When reviewing this MR, I noticed something which might be a follow-up issue:

  Hovering over buttons/icons gives them a darker grey background color. While some of these boxes are completely round (icons in the image panel), some have a border radius (like the language switch) and others have sharp corners (like the buttons for toggling the panels). I think it would be great to have a consistent concept behind this (or at least one I'm able to grasp 😉 )

  @nwindis what do you think?