On my side, I got it working on 2 different windows 10 and 11 with no issues.
And just for extra information, on windows to actually see your changes, you have to completely close Joplin and open it again. That means that you also have to close it from the taskbar as the program is still running in the background if you just do the normal ALT + F4. For the columns part, just comment or remove the userstyle.css -> line 86: "column-width: 30rem;" and that should work for you.