Consider tweaking colors to draw eye to differences

Thanks so much for the kaocha, I am really enjoying using it!

Some of my integration tests include the comparison of largish data structures. It can take my eyeballs a while to find the diff between expected and actual in these data structures.

I do sometimes use the search feature in my terminal to search for + and - and that can help and I was thinking focusing only on the differences, as #13 suggests, would help.

But I am also thinking perhaps a different choice of colors might help. I like the choice of colors chosen for differences, but I find that the red can take a while to spot because it is already used in non-diff syntax highlighting.

My initial thoughts are:

1. change the red to something else in default syntax highlighting, maybe blue.
2. take advantage of background colors to really draw the eye to changes. We might even be able to keep the normal syntax highlighting foreground colors with this scheme.

I am happy to give some color schemes a whirl for review, then a PR with changes, if there is interest in this idea.

No file or test is named in the issue. Locate the code that applies syntax-highlighting and diff colors, then review the existing output and any related tests; done means an agreed color treatment makes changes easier to spot without breaking normal syntax highlighting.