Some themes have gdm support, others don't. As far as I've been able to find, however, it's not possible to change the default colours of the gdm in Gnome. I do know that the default colour in Gnome is blue and Manjaro has edited it to be green. But is it also possible to pick a custom colour somehow?
great question! I tried to change something in the GDM theme and now can't change it back. Would love to know where these files are stored and configured as well.
Gnome uses *.gresource file for GDM theme. Needs to be complied properly, otherwise will fail. Also has to be compatible with the current Shell version, otherwise will fail.
manjaro-gdm-theme uses the upstream default theme, with some personal touch-ups and colors. More about it here
At install it stores in /usr/share/themes/manjaro-gdm-themegnome-shell-theme.gresource
If you want the default GDM Gnome theme, with the blue colors, then remove that package and reinstall
Do not confuse it with user themes ! While in your session can still change the shell theme with one you prefer, GDM still runs with the installed gdm theme. Do not manually replace *.gresource files as not all are properly made for GDM.
To create your own theme is another story. You can read the old topic here
We decided in the end to approach things the Gnome way tho. Also see the linked topics from it.