Use GM_addStyle to create the new class 'ccc', with the CSS attributes
you want.
GM_addStyle(".ccc { visibility: hidden; }");
Assuming myDiv is the element you want to add this new class to...
myDiv.classList.add('ccc');
or if you want to just flip the visibility state, regardless of what it
currently is, use "toggle" instead of "add" on the classList.