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 https://docs.mendix.com/howto/front-end/