Before start creating new theme you should know that it is always possible to change look of your carousel by overriding selected theme css at the 'Advanced Carousel Design'. But if you like to change the appearance this is how to create a new theme:
1- Theme css files located at the "\packages\whale_bootstrap_carousel\css\wbc_themes\". you can see currently there are one theme (default) there, so you can duplicate this folder and rename to something like "new_theme".
2- Now go to ''\packages\whale_bootstrap_carousel\controllers\single_page\dashboard\files\whale_bootstrap_carousel.php" and open file.
At "on_start" method, you can see an array that contains list of themes:
$this->carouselThemes = array('default'=>t('Default'));
You should add you new theme folder name (as a handle) and title tothis array:
$this->carouselThemes = array('default'=>t('Default'), 'new_theme'=>t('New Theme'));
Now at the dashboard carousel generator you can see new theme available for user to select.
3- If you want to new_theme be default theme, you should edit public variable and replace it with new_theme
public $carouselTheme = 'default'; //default value
4- Now you can start edit new theme css file.