I think what happens here is that you have Amber selected as your personal dark color scheme.
Some themes, like this brand theme, are not color scheme agnostic, it doesn’t make sense to apply a different color scheme to them. However, there’s no option right now to disable user color schemes for a specific theme.
So far I only explicitly declared accent color values right on the theme, so they don’t come from the color scheme. But with Amber, I guess grey background values are mixed into that. A hotfix would be to manually declare all color variables on the theme and not infer any from the color scheme at all.
My general preference would be an option to disable custom color schemes on specific themes, like brand ones.