Bug:
If there is no 'themes' directory inside '/boot/grub/' then the
Install script copies the contents of 'Vimix' directory to the root of
'/boot/grub/themes' directory.
Changes:
Create 'themes' directory if not exists.
Copied the 'has_command' function from the install script of Vimix to
the install script of Stylish Dark.
Also made some documentation changes and decorations :)
