On CachyOS 6.15.7-3 with GTK 4 1:4.18.6-1.1. I installed with pacman. Wofi would refuse to open and running in terminal would give a GTK-Critical error saying:
(wofi:1618): Gtk-WARNING **: 11:54:56.034: Theme parsing error: :32:18: Not using units is deprecated. Assuming 'px'.
(wofi:1618): Gtk-WARNING **: 11:54:56.034: Theme parsing error: :57:18: Not using units is deprecated. Assuming 'px'.
Fontconfig warning: using without calling FcInit()
(wofi:1618): Gtk-CRITICAL **: 11:54:56.056: gtk_window_resize: assertion 'height > 0' failed
Gdk-Message: 11:54:56.064: Error flushing display: Protocol error
Renaming the config file so just the style.css is used allows wofi to start.
Downgrading to the latest 1.4 fixes the issue and wofi starts normally with both the config and style.css.
I am using this rice that I found: https://github.com/diinki/diinki-retrofuture/tree/main/config/wofi
I can run gdb and send the log if it helps.