oh-my-zsh not applying themes

Posting here in case it helps others. The problem was that I was loading the theme after source $ZSH/oh-my-zsh.sh, so clearly oh-my-zsh wasn't loading it. (The instructions in the .zshrc template generated by oh-my-zsh are a little confusing. On a quick read they suggest that themes should be loaded below the User configuration line.)

For Arch-linux users: I had to un-install the grml-zsh-config package and reboot: yay -Rncs grml-zsh-config