I loaded Connections on my test blogs and the URL to the title appearing in the custom image header is clickable and does return one to the front page. In fact almost every theme if not every theme at wordpress.com does have a clickable header.
If you have removed the Blog Title and/or Tagline from here > Settings > General then be aware that doing that guarantees search spider bots will not find them and your blog will not be indexed.
CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM. You can go to Appearance > Edit CSS. There wordpress has provided a preview function.
Since there is no official Staff support for the CSS upgrade, and precious little Volunteer support, you must be patient while waiting for them to get to this thread. .Be sure to read this article before starting your editing > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/
P.S. I have flagged this thread so it gets moved to the CSS Forum for you.