New Tabs Page: Enable NTP to be theme aware

[imptrx]

Closes brave/brave-browser#5014
Themes/Styling were pre-built in an earlier PR for storybook (brave/brave-ui#485)
Quick view:

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protection-Mode
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

1 Set theme settings to 'Light'/'Dark'/'Same as OSX' from brave://settings
2 Open NTP's customization menu
3 Menu should be themed accordingly
4. Menu should dynamically update theme when if "Same as OS theme"* is set

*Assuming native OS supports dark mode - otherwise this option wouldn't be available from settings

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[petemill]

++ 🎨 🕶 🌡

[petemill]

CI didn't run because it was marked draft, and some reason hasn't re-run automatically, so I'll press the button now.

[bsclifton]

😎👌

[imptrx]

Rebasing as latest Jenkins build failed due to a version mismatch

[cezaraugusto]

LGTM

[cezaraugusto]

triggered a new rebuild

[cezaraugusto]

changing milestones as this work landed only in 0.70.x