Restyle changes header and file tree

Problem to solve

First pass on #31300 (closed)

Proposal

File tree shown	File tree collapsed

1. Remove background and border from the “top bar”, above the files. And tweak its padding/margins to account for a lack of background.
2. Change text “Changes betweenCompare source and target”
3. Change file browser toggle icon
4. Remove icons from the “X files +XXX -YYY” and use different text colors
5. Remove icons from the “+XXX -YYY”, change regular font weight, and use different text colors
6. Change file browser search input placeholder text
7. Change folder icon color to be lighter
8. Add file type icons (not sure if this needs backend)
9. Align file modification icon to the right (modified/added/deleted)

Links / references