Here is the recent post I mentioned above:
I agree that searching the forum is the best starting point.
I also agree that the search function leaves something to be desired... I struggle with it, too. Finding that thread above meant I did not use the search function... but instead, scrolled through all threads in the date range of when my memory said it was posted.
There is another thread that another user posted about using Dark Themes for GDM...
Can you check if you have either of these packages installed:
xserver-xorg-input-libinput
xserver-xorg-input-synaptics