Poipoi555, your not giving much information by just saying "doesn't work", as I have it installed in Opera version: 73.0.3856.344 (64 bit), it works fine.
...but, here is some information to help everyone understand the use of extensions.
Conflicts with Other Extensions?
If you have other extensions installed, deactivate them and check them one by one to see if any one of them interferes with Stylus.
Some Web Pages block Stylus
Some web pages may conflict with, or block Stylus since Stylus is a script itself for it to work. There was one time I had to deactivate Stylus because the web page conflicted with it. Nothing can be done about this. All you can do in this case is work the best with it.
Built-in Browser Pages
Stylus will not work in built-in pages of Opera such as the Speed Dial page or a default Tab page. Most extensions that affect web pages will not work in browser built-in pages.
If this is the case, click on the Stylus icon on the browser top bar and you will see this:
Stylus doesn't work on pages like this. As a security precaution, the browser prohibits extensions from affecting its built-in pages (like chrome://version, the standard new tab page as of Chrome 61, about:addons, and so on) as well as other extensions' pages. Each browser also restricts access to its own extensions gallery (like Chrome Web Store or AMO).
Hope that helps.