Simplify base16-shell `set_shell_theme` shell functions and calls.
Simplify base16-shell `set_shell_theme` shell functions and calls.

The distinction between 'dark' and 'light' themes appears to have been
removed from base16-shell and themes are now specified simply by name.
Therefore, remove the `set_[dark|light]_shell_theme()` functions and
use `set_shell_theme` directly.

alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles