Improve visibility on the git panel

[italomaia-bk]

Currently the git panel does not work with you to highlight what is going on:

• the diff does not have enough contrast; it is difficult to see what is being changed, even with different themes
• even though it is helpful to have all file diffs accessible and editable with a scrolling frame, it is sometimes confusing to see which file you're looking at; maybe make it clearer where one file ends and another begins, or add some kind of friction when changing between files
• the file-tree showing the changed files uses a checkbox; it is innovative and all, but maybe something more visually appealing would be better?