The easiest way to do this would be to open up the widget and look at the defined classes in the widget CSS
- Go to your local workspace (e.g. C:\Users\Hanna\YourMendixproject-main
- Go to widgets
- Extract the widget with 7zip (or similar tool, the mpk file is just a different extension for a zipped folder) (note: extract to a location outside your mendix project, like your desktop or documents, to prevent 2 instances of the same widget in the mendix\widgets folder)
- Open the extracted folder, go to widget\ui
- Open the widgetname.css file
Those are the classes that are defined specifically for the widget