Hi Rich,
Thank you for writing in, commonly this issue is caused by a syntax error somewhere on your Theme Options > CSS, Header > CSS, Footer CSS, or Element CSS on any of your elements in the header, content, or footer.
Please check all those mentioned custom CSS areas and make sure there is no typo on them. You can use this tool for checking CSS.
Sorry, but I am not entirely certain what do you mean by “weird that it’s not part of cornerstone yet” Breadcrumbs element can be used in Cornerstone and like other elements you can configure and style it without custom CSS.
I also advise to not use custom CSS (or any customazation) heavily as this can cause a conflict or make things complicated in the future (hard to maintain).
Hope it helps,
Cheers!