You can also use the Flatpak version:
I've been using the Debian packages supplied by Ubuntuzilla (despite their being hosted on gag Sourceforge): https://sourceforge.net/projects/ubuntuzilla/
Re: terrible look, I face the same thing with Telegram (the mouse reverts to whatever is the default in Qt and ignores the system settings). I haven't been able to fix that.
sudo flatpak override --env XCURSOR_THEME=DMZ-White
mkdir -p ~/.icons/default/cursors/
cp -R /usr/share/icons/DMZ-White/cursors/. ~/.icons/default/cursors/