Extensions and themes

Hi!
Im started to customize a Ubuntu 20.04 base.
i want to pu some extensions and themes in the iso, which are starts automatically for all users. do i have to use etc/skel folder or is it a better way?

EDIT:
i dropped dconf/user file in .config folder
extensions, themes, icons etc in the right folders.
after the first boot, everything is in place except extensions, but when i install system, everything is perfect.

sorry for my bad english :(