If you want to back to previous version of the theme you have to edit version.php file inside alpha.zip
Change plugin version number from 01 to 02 eg.
$plugin->version = 2019032801; // YYYYMMDDVV.
$plugin->version = 2019032802; // YYYYMMDDVV.
Now just install the theme (Compress the alpha’ folder if you use Moodle Plugin Installer).
