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.