Qt apps do not respect cursor theme under Xorg

Other solutions involve installing Plasma integration, but this would pull in most of KDE.

Fortunately, Qt apps still respect X resources. Add this to your login profile (e.g. ~/.profile, ~/.bash_profile, ~/.xinitrc):

sh
export XCURSOR_THEME="$(awk -F= '/gtk-cursor-theme-name=/ {print $2}' "$XDG_CONFIG_HOME/gtk-3.0/settings.ini")"
xrdb -merge <<<"Xcursor.theme: $XCURSOR_THEME"

Additionally you can wrap your theme changer so it does this after every change, or wait for file changes:

sh
#!/bin/bash

conf="$XDG_CONFIG_HOME/gtk-3.0/settings.ini"

while :; do
    inotifywait -e modify "$conf"
    xrdb -merge <<<"Xcursor.theme: $(awk -F= '/gtk-cursor-theme-name=/ {print $2}' "$conf")"
done

Note that you cannot change environment variables after login, but in my experience the xrdb method is enough for both Qt5 and Qt6.