Adding Custom Css rules can be done by navigating the Theme Options -> Custom Css section.
Paste here all the css rules needed but make sure the syntax is correct, otherwise you may break the entire site’s styling.
Custom Css rules can also be added individually for a certain page, using the WP Bakery Page Builder -> Page Settings.
Edit your page using the WP Bakery Page Builder, and from the Top Right corner of your editing window -> click on the Cogs icon.
In the new pop-up window you can start adding custom css rules, which will only affect the content of the current page, unlike the Theme Options -> Custom Css section, which will affect your entire site.