Support built-in vega themes

See vega-themes. Themes should be supported via the current theme infrastructure, maybe something like this:

alt.themes.enable('vega.themes.dark')

We’ll have to think about how to best populate the list of available themes, and how to make this work cleanly with user-specified themes from within Altair.

[domoritz]

I made a little demo at https://beta.observablehq.com/@domoritz/vega-themes-demo. The themes are definitely not tuned yet but having the infrastructure in place will be useful in the future and I hope that we get people to help with improving the themes.

[domoritz]

Update: Vega-Embed now supports userMeta.embedOptions.

What is the rational for that? Wouldn’t it be much easier if the theme choice is just part of the spec properly?

Themes are still developing and have not reached the maturity of Vega and Vega-Lite. You can always bake a theme into the spec as a config. Right now, Vega and Vega-Lite have no dependency on Vega-Themes and I think that’s a good thing at this point.

I think I would prefer something less obscure, why not go with @jakevdp’s suggestion of “config”: “theme”?

usermeta is already a supported property in Vega and Vega-Lite so I put it in there. "config": "theme" would require updates to Vega and Vega-Lite and also is less flexible compared to supporting all embed options.