Can you build a cutom UI in Mendix?

Is it possible to create/build a custom user interface for a particular client's needs or are you restricted to only use the built-in templates in Mendix? I am a beginner so bear with me if my question seems foolish.  Sincerely,
1 answers

Yes. Mendix actually did this by creating the module AtlasUI, which is part of any new project. You can create your own version of that and modify the templates to the customer's liking. Check out Project->App Store Modules->Atlas_UI_Resources.

To modify color, size, etc of the UI-elements, Mendix has provided means for that too. Check out Project→ Show project directory in Explorer. Go to theme. Open that folder in Visual Studio Code. You can add custom settings by adjusting theme\Styles\web\sass\_custom.scss and ~\custom-variables.scss. Or if you are working with native app: theme\styles\native\app\_custom.js and ~\custom-variables.js

Also, for detailed information check