updated documentation about theme-related commands

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
updated readme to reflect explicit dark theme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
updated dark theme names to reflect explicit configuration of dark theme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
update install script to point to explicit dark theme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
make sure the theme-changing command works on the explicit dark theme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
changed the output of install script confirmation to reflect both themes that are offered

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
Merged changes to make dark theme explicit (ie: not theme=zero or zero-light; but theme=zero-dark or zero-light)