[Site Design Revamp] Main View - Hide layouts row separators

[ovitrif]

Fixes #16487

Hides the gray lines (separators) between the categories on the Site theme screen to better align with the Android Material Design look & feel.

The Page Layout Picker (visible when creating a new page) is still showing the gray separators.

To test:

Theme Picker

1. Start the site creation flow
2. Pick an intent (or skip)
3. Enter a site name (or skip)
4. Expect to land on the theme selection screen
   • Expect no gray lines separating the rows of theme categories

Page Picker

1. Navigate to the page list
2. Tap the FAB to create a new page
3. Expect to land on the layout selection screen
   • Expect gray lines separating the rows of layout categories

Previews

Theme Picker	Page Picker

Regression Notes

1. Potential unintended areas of impact
   Page layout picker.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manual testing both the theme picker and the page layout picker.

3. What automated tests I added (or what prevented me from doing so)
   None - the UI changes imply no unit tests coverage.

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.

[antonis]

Great work @ovitrif 👍
The code looks good and works as expected 🎉
Thank you for handling this 🙇