The custom cursor for example.
I have looked around but I couldn't find a way to do that much customization.
Harman Kamboj the itch.io wizard magicked it into existence
for those wondering and aren't satisfied with ":the itch.io wizard magicked it into existence" itch.io allows you to write html directly when setting up a jam page instead of having the editor do it for you. This apparently also allows you to inject css.
The cursor has some explanation here https://css-tricks.com/using-css-cursors/ , no js required :)
CSS: https://developer.mozilla.org/en-US/docs/Web/CSS
HTML: https://developer.mozilla.org/en-US/docs/Web/HTML
Have fun jamming :)