Libadwaita hasn't crippled that, it's still very possible to load CSS. You just can't use the gtk-theme-name setting anymore.

Maybe it would be better if somebody fixed that thing with the menus. But I think IIRC if you wanted transparency in old X11 without a compositor, the toolkit had to copy the region under the popup, and then blit it to the popup... Yes, this meant that content in the transparent region that changed would be stale until the popup went away. So someone would have to decide if that would be a viable solution, or maybe someone could get away with just using the existing XShape stuff in GDK somehow.

How can I load css as a user?

As for the solution, why not remove shadows for non-composing wms?

Any CSS you put in ~/.config/gtk-4.0/gtk.css will load, you can put an @import statement there to load from another file.

The issue is not the shadows, the popup window itself is shaped. All that black region is supposed to be transparent. To fix with CSS you'd have to get rid of the "arrow" and make the corners square. But that may reduce usability of the popover since it no longer "points" to the parent. Maybe it could be positioned inside the popup somehow.

Thanks, will check the css solution.

You may also want to update your setup or use a compositor like picom if you want any kind of real transparency effects, X11 compositing is about 15 years old now so I think not having it would qualify as "extremely out of date" in software years...

I am not really a fan of transparency and other effects.

The option is there for you if you need it, at some point you may find that apps require it, and trying to hack around it could be impractical.