Introduce new high contrast theme

[jacksonhuether]

The new Altair update changed the default light theme, and the contrast on this new theme is very low.

It would be cool to have a new theme called high contrast, similar to what GitHub has, that makes it super clear to see the distinction between components and areas of the Altair user interface.

Maybe the high contract theme would just be the old light theme.

Any thoughts on this?

Nonetheless, thanks for the great open source tool.

[imolorhe]

Do you have specifics? I'd rather want to make the existing theme approach good enough if possible