@krinkle@mastodon.technology I don't know of anyone using difftastic by default, including me! (I use it >50% of the time though.)
I toyed with background colours but I didn't find anything that I really liked. Contrast is hard, and depends on the user's theme.
Background colours look bad with syntax highlighting (e.g. red background with blue comment text).
It also looked silly due to ignoring whitespace between symbols.
It's an interesting space and I'm still experimenting :)