Hello,
You’re using the Educon template, which includes some overridden add-ons, such as the feature box add-on. The grey background is coming from this override, which is why your CSS isn’t affecting the add-on.
If you want to design your site from scratch, you can delete this overridden add-on and use the core feature box add-on instead. (Remember, If you use existing layout, then no need to delete, Becuase, it will break your design).
Path: templates/shaper_educon/sppagebuilder/addons/feature
Thanks!