from looking at the KDE cursor documentation; it seems the file type is .cursor though it had a lot of steps involved as it shows you how to create a cursor theme from scratch, it requires you to have linux installed i believe (at least during some part of the process). I'd say it's not worth the effort for you, i'll have to figure it out someday when i do actually move my personal machine to linux. with that being said thank you anyway for your interest in wanting to help. as for your suggestion for a browser extension to use them—i will give it a try as soon as i get the chance thank you