Force background colors for line differences when "Syntax Coloring" is turned off

Turning off "Syntax Coloring" in the Difference view Preferences renders the web page much more responsive. But that comes with the price that diff lines (those starting with '+' and '-') do not have green/red backgrounds.

A hack to re-introduce those background colors is to add following declarations into the .patchContentTable .diffTextINSERT classes:

background: #dfd;
border-bottom: 1px solid #dfd;

... the following into the .diffTextDELETE class:

background: #fee;
border-bottom: 1px solid #fee;

I was not able to understand where the .css file lies on the server. The css is inlined in the main html file of the diff view page. (main html, because that page has several iframes).

How to actually insert those css declarations into the css is not cclear to me. Perhaps that requires some hacking of the java sources.