libhandy CSS is loaded too late
It's possible to have a window before the css has loaded, in which case the first few frames the window will be visibly unstyled:
This is a regression from https://source.puri.sm/Librem5/libhandy/commit/19c36ab55f6281a825b69a573c95b87b804e203b and related commits.
I can reproduce it in https://gitlab.gnome.org/GNOME/gnome-games/-/merge_requests/363