Buttons: outline style in editor don't have a background

[admturner]

Description

In the block editor, buttons with the "outline" style have a transparent background. On a white background this works as expected, but on any other color background the button appears wrong, and is sometimes unreadable.

Step-by-step reproduction instructions

Expected behaviour

The outline style button should consistently have a white background.

Actual behaviour

It has a transparent background.

WordPress information

• WordPress version: 5.7.2 and 5.8-beta2
• Are all plugins except HRS Blocks deactivated? No
• Are you using the HRS child theme? Yes

Device information

• Device: Desktop
• Operating system: Windows 10
• Browser: Firefox