Dark button text can look pretty bad in context

[ajb]

and https://github.com/dobtco/screendoor-v2/commit/c135c3385297ceb66d71c368768cd2febac61672

Not sure if I have any great ideas, but I think this looks pretty darn crappy.

[jrubenoff]

• Look at Bourbon's button mixin
• Let's try making every button color light and / or dark