refactor: remove safe-color-change mixin

[andrewseguin]

Converge on using get-theme-color to get theme colors with opacity. The missing piece was that get-theme color wasn't variable-friendly, so now it returns a color-mix if the color is not a "color"

Also, when possible, just use the foreground base color instead of various different colors when passing any opacity

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}