Reader: buttons block background is missing

[designsimply]

Steps to reproduce:

1. Create a new post.
2. Add a paragraph.
3. Add a button block.
4. Open the block settings.
5. Change the border radius.
6. Add a URL.
7. Toggle "Open in new tab" on.
8. Publish the post.
9. View the post in the Reader.

Result: the button background color doesn't display in the Reader at all.

Reader in app	Safari on iOS

^{Tested with WPiOS 14.6.0.1 TestFlight beta on iPhone 11 iOS 13.4.1 Dark Mode.}

Note: the Button Block was shipped in wordpress-mobile/gutenberg-mobile#1866 and it will become a component of the newer Buttons Block which is being worked on in wordpress-mobile/gutenberg-mobile#1866 — noting it because the html is subject to change and we'll also want to re-test once the Buttons Block ships.

Also reported for Android: wordpress-mobile/WordPress-Android#11679.

[designsimply]

The buttons block was re-tested in 14.8 beta testing and it was found that buttons still aren't displayed correctly in the Reader.

[leandroalonso]

Fixed in Automattic/wp-calypso#43779