When it comes to CSS, working in browser preview is not easy. So please email me in private your site credentials. Will work on CSS and then, if there is something we need to review as core update option, we will.
But there are settings in theme, like menu, which are forced to a width. And when you have more than the design, you need workarounds which can’t be all core.
But I can help override with child theme style.css and you don’t lose a thing.