how to customize boot menu and login page
installing new icons and themes
Here is an article from Fedora Magazine on boot themes
and here is a comment on changing the login page
Modern gnome-shell themes are stored as .gresource files (/usr/share/gnome-shell/gnome-shell-theme.gresource). Most modern Gtk themes (eg. Materia (formelly Flat-Plat), Pop, etc.) provides .gresource files that can replace default theme. Install them on /usr/share/themes, replace the default theme with:
sudo cp -b /usr/share/themes//gnome-shell/gnome-shell-theme.gresource /usr/share/gnome-shell/gnome-shell-theme.gresource
and finally restart GDM.