Offhand, it would seem commenting out the custom CSS ought to work. But it's not surprising that it doesn't. So, yes, reinstalling to theme or modifying the database are reasonable things to try in getting the site home page to work again. It's probably not worth trying to figure precisely what's going on, unless simple recovery methods fail.
I can't resist answering the question of what do instead to experiment with a theme's CSS. At least two possibilities:
- for very momentary experiments, use Chrome's Developer Tools. (Firefox has similar tools, but I don't know that Internet Explorer or Edge do.)
- for somewhat less momentary experiments, use a child theme. In it, you can have an ordinary
style.css file, in which you can comment-out parts as usual, or keep multiple versions on hand, or use git, etc.