Improve the podcast page background color

[geekygecko]

Description

The background colors on the podcast page didn't look right in the dark theme so I have improved them. I have also changed the Plus text from white to black.

Testing Instructions

1. Change the theme to dark
2. Subscribe to a podcast with only a few episodes such as "How to do everything" by NPR
3. Open a podcast page
4. ✅ Verify the loading background is the same color as the final page
5. ✅ Verify the bottom of the episodes and bookmarks tab are the same color

Screenshots

Before	After

Other themes

Checklist

• [x] If this is a user-facing change, I have added an entry in CHANGELOG.md
• [x] Ensure the linter passes (./gradlew spotlessApply to automatically apply formatting/linting)
• [x] I have considered whether it makes sense to add tests for my changes
• [x] All strings that need to be localized are in modules/services/localization/src/main/res/values/strings.xml
• [x] Any jetpack compose components I added or changed are covered by compose previews
• [x] I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

I have tested any UI changes...

• [x] with different themes
• [x] with a landscape orientation
• [x] with the device set to have a large display and font size
• [x] for accessibility with TalkBack

[wpmobilebot]

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

	App Name	📱 Mobile
	Build Type	debugProd
	Commit	540161cb0a2cbccd129680256c8e26129a636bf0
	Direct Download	pocketcasts-app-prototype-build-pr2811-540161c.apk

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

	App Name	🚗 Automotive
	Build Type	debugProd
	Commit	540161cb0a2cbccd129680256c8e26129a636bf0
	Direct Download	pocketcasts-automotive-prototype-build-pr2811-540161c.apk

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

	App Name	⌚ Wear
	Build Type	debugProd
	Commit	540161cb0a2cbccd129680256c8e26129a636bf0
	Direct Download	pocketcasts-wear-prototype-build-pr2811-540161c.apk