Fix dark mode issues

[atorresveiga]

Part of: #12281

Description

This PR fixes two small issues in dark mode.

The first one is the purchase button elevation. In dark mode, the bottom section elevation differs from the bottom sheet, making the UI look weird. In this PR, I fix that by using different elevations depending on whether the device is in dark mode or expanded.

The second issue was the selected rate background in dark mode.

Testing information

1. Create an Order with the processing status on a store that can create shipping labels
2. Tap on Orders
3. Tap on the order created on step 1
4. Tap on create shipping labels
5. Check that the shipment details section works as expected
6. Tap on a shipping rate
7. Check that the selected background is the expected
8. Turn on dark mode
9. Check the UI is readable and that the shipment section and the purchase button have the same elevation

The tests that have been performed

• Testing the UI works as expected

Images/gif

Before	After

• I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on big (tablet) and small (phone) in case of UI changes, and no regressions are added.

[wpmobilebot]

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

	App Name	WooCommerce-Wear Android
	Platform	⌚️ Wear OS
	Flavor	Jalapeno
	Build Type	Debug
	Commit	8d591c1
	Direct Download	woocommerce-wear-prototype-build-pr12966-8d591c1.apk

[wpmobilebot]

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

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	8d591c1
	Direct Download	woocommerce-prototype-build-pr12966-8d591c1.apk

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 39.73%. Comparing base (16feecf) to head (8d591c1).
Report is 15 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...i/orders/wooshippinglabels/ShippingRatesSection.kt	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk   #12966      +/-   ##
============================================
- Coverage     39.73%   39.73%   -0.01%     
  Complexity     5983     5983              
============================================
  Files          1267     1267              
  Lines         73223    73224       +1     
  Branches      10041    10042       +1     
============================================
  Hits          29094    29094              
- Misses        41563    41564       +1     
  Partials       2566     2566              

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