WhatsApp Block: In the Twenty Twenty-One theme, the focus style overrides the icon in the WhatsApp button

[andrewserong]

Steps to reproduce the issue

1. Activate the Twenty Twenty-One theme on your test site
2. Add the WhatsApp button
3. Click on the WhatsApp button on the front end of your site
4. It will appear as though the icon in the button has disappeared

What I expected

The icon to remain white

What happened instead

The icon disappears (really the background value gets overridden)

Screenshots

It should look like:

It appears to be caused by the background attribute of this rule from twentytwentyone:

So, we might want to increase the specificity of the a.whatsapp-block-button selector by adding a :hover pseudo class.