Allow overriding just the theme with a provider

[holic]

I have a site with a few different designs/color schemes depending on the subpath you're on. It would be great if I could just override the theme with a provider rather than redefining the whole RainbowKitProvider. I could probably wire up my own component that wraps RainbowKitProvider with some default values, but thought I'd see if anyone else wanted this feature.