Re: Stylizing the navigation bar
I’m not really sure what you are asking:
Do you want to allow all blogs to have their own choice of theme, but ensure that they use the same font for the admin bar?
That’s not really possible, because you don’t have any control over the CSS files in the sub site themes – if they define their own CSS settings for the admin bar / standard fonts / etc., they are the ones that will be applied in their site.
If you want that degree of control, you probably need to create a small set of bespoke themes that they can choose from, ensuring you have set the admin bar settings as you want them in each of those themes.