It's an oh-my-zsh plugin, choses a random theme for each session. If you not like the chosen theme you can run randeme_rm to never show that theme again.
git clone https://github.com/ex-surreal/randeme.git ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/plugins/randeme
And add the plugin in ~/.zshrc
plugins=(... randeme)
NOTE: You have to source the ~/.zshrc to see changes