Reading Preferences: Fix customization sheet

[dvdchr]

Part of #22925

In this PR, several improvements are included for the customization sheet, which should also address the issues raised in #22851 (review):

• The navigation bar button now adapts to the currently highlighted color theme.
• Added decent/passable support for landscape mode.
• Added accessibility support.
• Added h4x0r and Candy themes. 🎨
• Added a soft gradient at the bottom of the Preview section to indicate that there are more contents to scroll.
• Added the actual feedback survey link.

Portrait	Landscape

To test

• Launch the Jetpack app.
• Ensure that the Reader Customization flag is enabled.
• Go to the Reader tab and open any post.
• Tap the reading preferences button.
• 🔎 Test the screen with various configurations. Verify that the contents are all reachable, both in portrait and landscape mode.
• 🔎 Test with VoiceOver, and verify that the hierarchy of the customization sheet makes sense.

Regression Notes

1. Potential unintended areas of impact
   Should be none. This change is isolated to the customization sheet, which is a new component that's hidden behind a disabled feature flag.

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

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

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• 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.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22967-31d3599
	Version	24.6
	Bundle ID	org.wordpress.alpha
	Commit	31d3599
	App Center Build	WPiOS - One-Offs #9399

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22967-31d3599
	Version	24.6
	Bundle ID	com.jetpack.alpha
	Commit	31d3599
	App Center Build	jetpack-installable-builds #8443

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wargcm]

LGTM! 🚀