You can see this on the 2nd screen of the calculator tab in the GTS tool.
I don’t have GTS up, but right click on the text and it will show you where the colors come from.
Rejected due to dark mode… 
I’m very confused.
Color source is from Calculator - Unit Converter - Background Color.
IF you have set that color as a dark (or another color) but it shows up white in validation tests it’s a bug and there’s nothing you can do on your end (except completely ruin your color scheme to workaround the GTS bugs - which you shouldn’t do). You can submit this bug from an 1:1 inquiry through the Seller Portal with detailed explanation.
This bug is actually fully not-fixable. It’s only in dark mode, which we cannot even change with themes.
I think this particular one was fixed with the w2 editor, so just re-export / re-submit your theme and you’ll be okay.