I get emails with diffs in them every day, and Gmail does really horrible things to them. In the Gmail web UI diffs inside emails are nearly unreadable. I don’t know why they choose to mangle them in this way, but thankfully there is a work around: Beanstalk SVN Diff Colorizer for GMail.
I use Google Chrome as my main browser, so I installed this as a user script, clicked reload on my Gmail tab now diffs in emails are much easier to read again.
The CSS is very simple:
[sourcecode lang=”css”]
ins { background-color: #cfc; }
del { background-color: #fcc; }
[/sourcecode]