WordPress menu change classes

Hey @marcelkraan,

can i solve this in the custom CSS?

Custom CSS option is used for adding additional CSS code only.

However, you can follow the below article link regarding the process of adding custom classes in menu items:
https://sevenspark.com/how-to/how-to-add-a-custom-class-to-a-wordpress-menu-item

Hey @marcelkraan,
If the old CSS is overwriting your custom CSS code, then you can try adding !important in your stylings, this will give more weightage to your CSS code.

Ex:

.abc{
  color: red !important;
}

Article on CSS !important property : https://www.geeksforgeeks.org/how-to-apply-important-in-css/