Expand theme variables

[craigcook]

To support multiple brands, Protocol treats lots of property values as variables, mainly colors, fonts, and font sizes. But there are more things we can make themeable with some more variables. Main ones off the top of my head:

border-width
border-style
border-color
border-radius
box-shadow

And likely more I'm not thinking of at the moment. We could potentially assign theme variables for spacing (padding and margins) as well, but I feel like spacing is one place where we can enforce some cross-brand consistency.

[craigcook]

Thinking more about this, we should clarify that these aren't necessarily specific CSS properties, but aspects of a component's presentation that should be consistent across all instances of that component under a single theme.

For example, the rounded corners of buttons might be defined as a theme variable called button-border-radius and that value might differ between brands, but ALL buttons within a given brand theme would have the same border radius. That way Firefox buttons might have a 6px radius and Mozilla buttons might have a 0 border radius.