Support theming to different colors

[wwwillchen]

After #663 we will support dark theming, however there's still not a way to theme to different colors (e.g. use a different primary color besides blue).

There's two options:

1. Support a fixed set of color palettes

We could create CSS files for different color schemes and serve them depending on what's configured. See: https://material.angular.io/guide/theming#multiple-themes-across-separate-files

The upside is that we could curate a small set of relatively good-looking color schemes. The downside is that there's only so many color schemes we would realistically support, particularly in terms of combinations.

2. Support dynamically setting the color palette

This would be quite a bit of effort unless Angular Material provided a simpler way of generating a color palette from a single color. See: angular/components#29104

https://github.com/angular/components/blob/main/src/material/core/tokens/m3/definitions/_md-sys-color.scss

This is similar, IIUC, to how Gradio does theming https://www.gradio.app/guides/theming-guide which extensively uses CSS variables, and in particular uses a few color CSS variables to generate variants for the rest.