There is no easy or "free" way to do what you want to do. 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.
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.
Please read this post > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/