DOC: add documentation of themes & customization

Hi, we’re using our own theme (config) for our charts using vega - if I understand correctly it can be done through the altair by using Chart.configure() for each chart.

What I am proposing is to add single method like

import altair as alt
altair.configure()  # or altair.defaults()

so that we won’t need to patch every single chart. I will be happy to PR (sounds trivial), if you think this is a good idea

[jakevdp]

It’s doable, it just will take some thought to implement well.

[joelostblom]

I am closing this as themes are now documented.