New blog Activation Message messed up

I'm wondering what wordpress and buddypress and/or theme files I need to review to figure out why the font of the activation notice and new blog link are messed up ("Congratulations" is way too big, and the link to the blog is using the main site ID number, not the real domain name, so it doesn't work as a link). I know how to alter buddypress files in my child theme, but not how to get child theme to alter wordpress core files, if that is possible. I'm using Blogs-mu as theme.