Allow themes in ZSH_CUSTOM folder

https://github.com/robbyrussell/oh-my-zsh/pull/525 added the possibility to use a folder outside of the oh-my-zsh repo to allow including oh-my-zsh as a submodule while keeping custom modifications. This path, if set, is currently not used for theme path resolution. This patch changes theme lookup from $ZSH/custom to $ZSH_CUSTOM, to allow for custom themes with oh-my-zsh as submodule.