Hi Niels,
Try this https://ux.mendix.com/theme-creator.html You can setup your basic Mendix template in the right colors and export it.
To import in the Modeler go to --> settings --> Model (tab) --> theme import.
Good luck!
To add another tip: use GapDebug to debug your iOS device (https://www.genuitec.com/products/gapdebug/)
I would check if the designer knows about SASS (sass-lang.com). Because the whole Mendix UI Framework is build using SASS. Here are some other links to get the styler started.