A new user to Backdrop asked the following question in Gitter today, it seems like a question that other might have, so I'm posting it here in the forum.
"Using the Backdrop default theme and need to make CSS changes. (Menus are HUGE, hero block has more padding than the presidential bank account.) Spelunking through inspector properties and CSS files, it appears some are programmatically generated, and I have no idea where to make edits. Help?"