add colors to buttons

[ryanatkn]

This has some rough edges but I'm trying to keep the size manageable.

future:

• thicker borders for selected states?
• figure out text color variables
• multiply/blend hover/active states (positioning ma
• think about color shade variants (maybe color_a_1-9 etc?)
• maybe both semantic_classes.css and utility_classes.css?
• other elements like .chip_1 etc
• figure out outline-offset in context of all elements (2px looks good for some things but will probably break a lot of expectations)
• .deselectable needs a visual hover state not just cursor
• bg/fg are inconsitent with their numbered versions, the former use tint, but we definitely don't want tint on the latter, blend modes would be correct