refactor!: Remove 'theme' props from components

[driskull]

Related Issue: #2189

Summary

This PR is the first step of removing the use of the theme attribute selector.

• Removes all 'theme' properties from components
• Removes all use of the theme properties in tests
• Adds css class selectors for light/dark themes

The next steps would be to

• Replace theme attribute selector with classes
• Update stories to replace 'theme' properties with classes.
• Update test apps to replace 'theme' properties with classes.
• Document the theme classes somehow

Please review

[paulcpederson]

nice! tons of work, thanks @driskull . I don't think we should be using getComputedStyle unless we absolutely have to, but other than that looks good to me

[driskull]

For now, closest will look for either the class or attribute. Once we remove all uses of the attribute we can update the utility to remove that part.

[paulcpederson]

I think you may be able to remove the theme name vars altogether?

LGTM, though