Make dark theme more useful

[antonok-edm]

Changes

This adds some generic color overrides that should be useful for most browser components in dark mode: text, panel colors, and button colors.

I've tweaked the Card component to use the panelBackground theme color since it's also required to be able to theme the webcompat reporter dialog.

Test plan

Link / storybook path to visual changes

Integration

• Does this contain changes to src/components or src/

  • Will you publish to npm immediately after this PR, or wait until sometime in the future?
  • Incompatible API change to something existing (major version increase)
  • Adding new backwards-compatible functionality? (minor version increase)
  • Fixing a bug backwards-compatibly? (patch version increase)

• Does this contain changes to src/features for brave-core?

  • Are there non backwards-compatible changes required for brave-core? Do not merge until brave-core PR is approvable. Link to brave-core PR:
  • Will you create brave-core PR to update to this commit after it is merged?
  • Wants uplift to brave-core feature branch?
    • When uplift-approved, merge to brave-core-0.VV.x feature branch
    • Create additional brave-core PRs for each feature branch to update commit

[petemill]

+1 🎨