This is an instruction on how adding a new theme for the 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.