fix(button): Default to primary color

[kfranqueiro]

Fixes #1345.

I should point out a couple of caveats with this approach, which differs from the prior approach of providing --primary and --accent (now AKA secondary) as opt-in classes...

• By setting a hard default to primary, light-vs-dark considerations are dropped, and in general, contrast considerations will be left to the user when customizing theme if customizing via CSS variables.
  • I'm a little concerned especially about flat buttons using primary for text/ink color, in cases where the primary color is relatively light.
• Current versions of Edge with (buggy) CSS variable support will have problems displaying this. I'm pondering looking into mdc-button in Edge browser is colorless #1102 to see if this can be addressed.

[kfranqueiro]

Added another commit to address contrast issues with dark theme (perfect example of one of the caveats above)... I'll definitely want to talk over dark theme with designers some more as to the direction we should take that.

[lynnmercier]

Thanks for doing this Ken. We'll discuss dark theme more in the next couple months