refactor(menu): Update base theme variables; rename theme variables
refactor(menu): Update base theme variables; rename theme variables

- Add appropriate focus & hover styles with a variant
- Update icon styles

BREAKING CHANGE: ([Detailed here](https://gist.github.com/kylegach/3f1300c071c8b47952eb12729e5a90e8#menuitem)) Rename theme variables: `MenuItem_color_text` to `MenuItem_color`, `MenuItemIcon_fill` to `MenuItemIcon_color`, `MenuItemSecondaryText_color_text` to `MenuItemSecondaryText_color`

mineral-ui/mineral-ui,mineral-ui/mineral-ui