Block Editor: Gray background color appears behind theme background color

[t-hamano]

Description

A gray background color is applied behind the theme background color in the post editor and site editor.

This background color appears to have been applied in #59334.

This issue does not occur with WP6.5, it occurs with the latest Gutenberg.

Step-by-step reproduction instructions

Open the post editor or site editor.

Screenshots, screen recording, code snippet

Post Editor

Site Editor

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[annezazu]

Noting that I'm not adding this to the 6.5 board since it doesn't occur within the release but let's make sure whatever is causing this isn't backported for the next beta.