If you want to add your own zsh theme, you can place it in ~/.zsh.prompts and it will automatically be picked up by the prompt loader.
Make sure you follow the naming convention of prompt_[name]_setup
touch ~/.zsh.prompts/prompt_mytheme_setup
See also the Prezto project for more info on themes.
If you want to customize your zsh experience, yadr provides two hooks via ~/.zsh.after/ and ~/.zsh.before/ directories.
In these directories, you can place files to customize things that load before and after other zsh customizations that come from ~/.yadr/zsh/*
For example, to override the theme, you can do something like this:
echo "prompt yourprompt" > ~/.zsh.after/prompt.zsh
Next time you load your shell, this file will be read and your prompt will be the youprompt prompt. Use prompt -l to see the available prompts.