[Site Creation] Tweak UI of Site Theme to match design specs

[ovitrif]

Tweaks the UI of the Site Theme screen to match the Figma Designs:

• Add divider on top of helper text footer
• Increase padding around helper text in footer to 16dp
• Increase font size of category title to 20sp

Small refactoring was also added to this PR, concerning code cleanup that we identified after merging #16584

To test:

1. Start Site Creation flow
2. Skip until the Choose a design/theme screen
3. Expect UI changes:
   • Font size of category titles increased
   • Padding around footer text slightly increased
   • Semi-transparent divider between last preview cards and footer text

Previews:

Category Titles Before	Category Titles After

Footer Before	Footer After

Regression Notes

1. Potential unintended areas of impact

   • Category titles font size on the Page layout picker (FAB on My Site → Site page)

2. What I did to test those areas of impact (or what existing automated tests I relied on)

   • Manual testing.

3. What automated tests I added (or what prevented me from doing so)

   • Non-functional UI changes are not in scope of automated tests.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-16619-build-124044.apk

• WordPress-pr-16619-build-124044.apk

[antonis]

Great work @ovitrif 👍
I've tested the implementation on a Pixel 5 / Android 12 and everything works as expected. The code also LGTM 🎉
I've just left a comment/suggestion but it is not blocking :)