While in the Kentico Xperience UI making edits, you may have to define specific classes to get those elements to look like you'd expect. While in the Kentico UI, right-click on one of the Xperience page builder icons/components and get the css class name from it. In your stylesheet, override that class by prefixing it with something in your design like body main.main .ktc-section and create your styles for it in your stylesheet.