Hi again,
The principle is simple with RW: in the inspector the page with HTML, CSS, Java, Prefix… is the place to install the code and in the inspector of the stack considered it is HTML or CSS box to indicate the name (class ID) of the CSS code to call and use.
To return to the case you gave at the beginning of the discussion with the modal, you are missing the classes (name and code) created by Adam Shiver for his tools. If one exists for what you want to do then if he communicates it to you you can modify it according to your needs. It is still necessary that the design of Foundry itself allows it. There I cannot inform you because I do not know.
In summary, creating css code and naming it is not of much help if it cannot be applied to the targeted object q.e.d. 