Support theming

[domoritz]

Add support for theming with Vega configs (https://vega.github.io/vega/docs/config/).

[josesaranda]

I think this support require a lot of effort. You can accept the PR for a minor version.

[domoritz]

I've actually manually set the config before to create a dark mode. It's actually pretty nice.

You can find my code at 8a9de43. However, I didn't extract the Vega config into the Falcon config, which I now realize is the right approach.

[josesaranda]

If you want to add more theming support you can do that, but i only need to change the bar colors at the moment. Please approve it

[domoritz]

Unfortunately, this change only would introduce backward-incompatible changes once we add support for theming. I don't plan to accept it into the main repo but you can use your fork until theming support lands here.