Change default find highlight color

[cmyr]

Currently find highlights are red by default. We should default to NSColor.findHighlightColor or something? There may also be a color defined in the theme that we can use. (The motivation for not using the colors in the theme is that we support multiple 'find groups' with different colors, and want these to be distinct, but I think we should default to an appropriate color for the first group.

[mohanio]

@cmyr , @nangtrongvuon added a fix for this here #481

[cmyr]

cool, thanks!

edit: will need to think about this a bit, but will give it a proper look monday or tuesday.