It seems I got sidetracked and took the CSP error failing to load surfaces.css from an Atlassian background script for our own 
So, back to square one. The issue seems to be with the surface detection itself.
Below is the screenshot from Automation for Jira. They are using the approach suggested by @MitchGavan, which we are using too, but that does not work anymore and does not resolve to the correct background color.
As @Archer suggested, using background-color: inherit seems to work though.
@DanielDelCore is inherit the way to go? I believe the issue is not yet resolved.
Update:
I just noticed this again and wanted to highlight that this issue only exists if you view an issue on the board (clicking an issue opens a modal). It works correctly on /browser/<issue-key>. The general assumption seems to be that modal dialogs always have a darker background color shade. However, this should not be the case if you view an issue in the context of a dialog.