Resolved Adjusting the height of Product Slider - Difference between custom.css and custom code of SP Page Builder

Hi, grateful if you could advise the difference between adding codes in custom.css and the custom code of SP Page Builder on changing the height of the Product Slider from 665 to 300

I added the code below in the custom.css but no effect has taken place.

.sppb-slideshow-products-item-bg {
height: 300px;
}

When I add the same code in the custom css of SP Page Builder, it was working at once.

The url of my site is http://cms.hkbws.org.hk/eshop

Regards,

Forrest