Hi Romir,
In the latest versions of mendix, the styling section is done in Properties of the elements.
Here, you can see In properties, go to styling section and you can find class input. I think you got it.
if your properties were not showing, then get it from View of Control bar. View > Properties.
Hello Romir,
You need to have the properties toolbar added to your mendix studio pro:
Styling in mendix is done with classes or with styling directly written on the element, more information can be found here;
https://docs.mendix.com/howto/front-end/customize-styling-new/
Check also out the learning paths related to styling:
Hope this helps
Good luck
The option has moved to Styling Properties. See below