1. Flatpaks missing dark GTK theme

flatpak install org.gtk.Gtk3theme.Adwaita-dark

Also see this article about it.

2. Virtual Manager (KVM) ethernet not working

Open this file and add the following at the end right under #firewall_backend = “nftables”.

firewall_backend  = "iptables"

Restart the libvirtd service.

sudo systemctl restart libvirtd

Now it should work after restarting the VM.

Source

3. NVIDIA drivers

If you have a NVIDIA card, I suggest you follow the steps covered in this tutorial created by Fady Osama.