Page Builder is Magento’s tool functionality for providing merchants a convenient way for designing pages ready for production; furthermore, allows faster releasing times and a cost reduction related to web development. 

Originally provided in the Magento Commerce / Enterprise version only, it is now also available in Magento Open Source, starting from version 2.4.3.  Page Builder provides a very intuitive drag and drop interface that also allows setting those little details that make a website stand out from the rest; being CSS one of them.

Adding a customized CSS style on any Page Builder element is as simple as adding a new HTML code element in the page.

Then, by editing the html code block, any custom class CSS style definition can be defined, just like in any regular html file.

After saving, the custom CSS class can be used in any other Page Builder block by just adding it into Advanced properties of any block setting

After saving again, the CSS class will be applied to any block that is referencing the custom CSS class.

However, in some special cases, a deeper knowledge of Magento theme development would be required for a whole customization of self-generated elements, but the general website development process experience will definitely be improved.