I'm not sure why it was added in first place, because there's no need of !important (which, as you can see, has its use discouraged for a good reason) to change text color of title and header in this instance, and there's also CSS prior to these snippets which's used to change text color for both without using !important. Either way, you just need to remove color:white !important to solve the issue.
Thanks! I've removed those lines of CSS, and it seems to work just fine now.
One thing I've also noticed: in the page preview, or when you first load a page, the border briefly appears as the default blue before switching to the custom color. This doesn't bother me too much since the Steven Universe Wiki seems to have the same issue, but I'm wondering if perhaps there might be a way to fix it?