Enable input focus state customization

Enable input focus state customization

- Add new mixin to generate and customize focus state as needed
- Adds variable to set default color
- Include clear disclaimer about customizing this—it's about users', so
don't go making everything bright red and expect them not to be
confused or alarmed.

Relevant issues: #2742, #4185, #7942, #8171, #8610, #9044