Johannes,
Hello!
I think the issue is related to the ID that is likely set in the [intense_menu] shortcode. If you have an ID set in the shortcode, you can remove it all together or set it to something that starts with letters. An ID cannot start with numbers. We automatically generate an ID if there is not one set in the shortcode. Our shortcode may generate one like “menu_20160219”. If you change the ID in the shortcode, the styles should be applied correctly.
Just for reference, a class on an HTML element can start with numbers, but an ID cannot. I don’t always remember this either. It took me a little bit to look over the code and figure out why the styles were not being applied.
Let me know if you have any further issues.
Thanks,
Josh