A11Y changes: better :focus styling, increased contrast, and added image alt texts

@talbs & @adampalay - here are the first three a11y changes from TPG. Let me know if you'd like to see the TPG issues from the report.

- Doubled up :hover and :focus styling to improve a11y
- Increase contrast of certain UI elements for improved a11y
- Added some image alt text for a11y