Styling Discourse elements for WCAG within one defined theme

I have a single “Default” Theme set up on my forum. I used the Custom CSS/HTML and Uploads controls to attach a dark background image to the #main-outlet div. This all works fine.

Now, a user has changed their Interface preferences, switching the Regular Color Scheme to “WCAG Light”. With these colors, the background image no longer makes sense. I would like to change it, but the theme-wide Custom CSS/HTML section is the only one I see under Customize.

Within my 1 default theme, how can I style an element differently depending on a user’s preferred Regular Color Scheme?