This is an instruction on how adding new theme for add-on:
1- Copy/paste one of the folders inside css/jcarousel_themes/
2- In the css file, rename all the parent classes to new theme name.
3- At the block main controller (whale_jcarousel\blocks\whale_jcarousel\controller.php), line 23, there is this variable name $themes, contains all the available themes titles. add your new theme title to this array:
$this->themes = array(t('tango'),t('light'),t('dark'), t('navy')); //tango is default jcarousel theme
Now you can see new theme at the block editing UI combo box.