If you place the caret on one of the lines and use ⌃⌘T (Bundes →
Select Bundle Item…) and switch to Settings then you should see that
there is a “Diff ▸ Style: Changed” style that sets the background/foreground (if you do not immediately see it, enter ‘background’ in the search field).
Oh wow! Thanks! Never knew that ⌃⌘T takes into account the caret position.