how does main.scss look?
You need to import the custom-variables like this:
From looking at the images you submitted, I *think* (without having the complete knowledge of your app's mapstructure, but considering basic mendix structure) you have gone back a map too far when importing.
So for a custom styling module I'm currently working on, the main.scss file (in your normal theme map, not the module's one) the import is as follows:
Also repeat that process for both your ‘custom-variables’ & ‘exclusion-variables’ files if you have custom code in those.
Other than that, in the comments I've already made note of the fact that the custom UI module should be at the top of the list when it comes to theme, like so: