fix: toast component color contrast

[runningonsideprojects]

Fixes #3857

Changes:

• moved toast-text-color and toast-background-color into the light, dark, and high contrast themes in __variables.scss
• updated .toast color and background-color to use @include themify {...} so the colors are configured depending on theme modees in __toast.scss
• updated .toast__close (the X button in the toast) to use the toast-text-color in __placeholder.scss.
  • this was not part of the issue description. i decided to do this because the icon-toast-icon-hover did not adhere to WCAG color contrast guidelines.
• removed icon-toast-icon-hover in __variables.scss

Screenshots:

Video showing the new toast colors with High Contrast and Dark Mode:

Screen.Recording.2026-02-12.at.7.25.56.PM.mov

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines