Dark Mode Theme and Toggle

[hawkishpolicy]

🗣 Description

Edited the existing CFThemeProvider to allow for quick switching between 'light' and 'dark' modes.

A new CFThemeContext.Provider is now wrapped around the MUI ThemeProvider.

A MUI Switch with necessary props and tooltips now sits at the end of the Header ToolBar.

💭 Motivation and context

Main motivation is to provide a more enjoyable user experience across the site.

🧪 Testing

Tested locally.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All future TODOs are captured in issues, which are referenced in code comments.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [ ] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [ ] Tests have been added and/or modified to cover the changes in this PR.
• [ ] All new and existing tests pass.

✅ Pre-merge checklist

• [ ] Revert dependencies to default branches.
• [ ] Finalize version.

✅ Post-merge checklist

• [ ] Create a release.