I install a theme using M-x package-install [return] doom-themes. And it installs the themes and I use the themes. However, when I close spacemacs and start it again, this package is gone and needs to be reinstalled again. How can I make it so that the doom-themes package is installed between spacemacs sessions?
dotspacemacs-additional-packages and put the package name inside that list. That way Spacemacs would know not to delete it on startup.