You failed to post the URL for the blog to which you are referring.Are your referring to a free blog that's being free hosted by wordpress.COM or not?
I think it’s important to know that wordpress.com is a WP-MU blogging platform. The themes here were all originally coded to run on free standing wordpress.ORG software installs. In order to run on wordpress.com software they were “adapted” and some features were eliminated or added. As this is a WPMU blogging platform we cannot access and edit the underlying php and html in the templates. The themes we use here are all derived from shared templates and that means every blog wearing the same theme is in essence sharing the same underlying template here at wordpress.COM. Therefore only Staff can edit the themes and templates underlying them.
We cannot add themes at wordpress.COM, we are limited to the themes found in the dashboard under Appearance > Themes only. If you have CSS editing experience, you can get the paid custom CSS upgrade and renew it annually. It will allow you to stylize the appearance of themes you find here > Appearance > Themes, but it does not allow you to change the functionality, by editing the underlying template. Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
Since there is no official Staff support for the CSS upgrade, and precious little Volunteer support, I recommend that you don’t buy the upgrade unless you have at least moderate understanding of HTML and CSS because you must be prepared to do the work on your own. If you are considering purchasing the upgrade, you can go to Appearance > Edit CSS. There wordpress has provided a preview function where you can try before you buy.