add focus states

[scottaohara]

add :focus selectors to <a> and <button> elements Set focus states for <a> and <button> elements to provide parity for mouse and keyboard interactions.

[claviska]

Good call — in Chrome/Firefox the default outlines appear on focus, but not in Safari and likely other browsers.

I'd prefer to add a proper focus state instead of reusing the hover state, and consistency across browsers is highly desirable. I really like how BS4 beta buttons handle this, but we need access to color modifiers to make it happen without setting arbitrary variable values.

[scottaohara]

Agreed, a unique focus state would be best, but I just went with the idea of "get something in there because reusing hover is better than nothing" rather than "let me try to guess what design aesthetic you would want" :)

totally cool with cancelling this PR if you want to design these yourself.

[claviska]

This was fixed yesterday in the buttons rework. Thanks for the heads up 👍