RI: Post Card UI - Use default material theme colors

[ashiagr]

This PR addresses feedback to

• Replace reader specific colors with our default ones (using Emphasis to get variations in the shades)
• Make the body/excerpt text color the same as the post title color

To test

• Launch app and go to Reader tab
• Notice that colors for texts, borders, backgrounds on the post cards look similar to the ones used earlier or are acceptable
  Reference: Figma -> Apps Reader Improvement -> Postcard UI -> postcard anatomy (android)

Note

Trying to figure out how to replace reader_post_list_background with a default one. This is under discussion, will update once we have any decision on it.

PR submission checklist:

• I have considered adding unit tests where possible.
• 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]

You can test the changes on this Pull Request by downloading the APK here.

[khaykov]

Looks great! Thank you so much for the changes 🙇

[mbshakti]

Looking good!!! Thanks a lot for making the changes @ashiagr