Add built-in themes

While the default theme tries to reproduce the look of the old [Dialog](https://en.wikipedia.org/wiki/Dialog_(software)) utility, it may not be to everyone's taste. While users can already define their own custom theme, offering a few reasonable presets could reduce the time it takes to get up and running.

I think a couple themes could be a good start (a "modern" theme, maybe a dark/light variant, ...). We could take existing vim/other themes as an inspiration for what color palette people like.