How can i unlock custom shell ? when in other gnome versions it can select the shell I want. I am using gnome version 43.1
Moderator edit: Moved query from topic title to first post
Welcome to the forum! 
You’ll need the User Themes extension enabled. It’s part of the gnome-shell-extensions package which should already be installed.
Also make sure to store any downloaded themes in ~/.local/share/gnome-shell/theme
Since he has a custom theme (looks like WhiteSur) installed, the matching GNOME Shell theme should already be in /usr/share/themes/<theme-name>/gnome-shell/ or ~/.localshare/themes/<theme-name>/gnome-shell/.
I have a Gnome install and the downloaded themes are in ~/.local/share/gnome-shell/theme/<themename>, looks like there are a few places they can go.