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Fixes #9180
Co-authored-by: James Eapen <jamespeapen@gmail.com>

The ZSH_THEME_RANDOM_BLACKLIST variable will be deprecated until it's
removed 2 months from now.

...preventing an infinite loop.

Co-authored-by: Fran Garcia <fran.miranda@gmail.com>

Also add comments and unset leftover variables, and print only the
name of the theme loaded.

When looking for $ZSH_CUSTOM themes, the chosen algorithm is to add
the theme names to the pool disregarding the path, and then source
whatever theme is selected with the same logic as the init script,
which is to source first custom themes even if there is another
default theme of the same name.
Co-authored-by: Mihai Serban <mihai.serban@gmail.com>

The statements for selecting a random theme in oh-my-zsh.sh and the themes
plugin are duplicate. Most people eventually settle on a theme, making those
lines in oh-my-zsh.sh superfluous. To address those, it may makes sense to put
the random theme functionality into a theme of its own (since themes are just
zsh scripts.