At wordpress.COM, we cannot modify the underlying theme files, which are written in PHP script. The reason for this is that this is a multi-user platform which means we all share the same underlying theme and wordpress core files. If one person were allowed to edit the theme files, it would change that theme for everyone using that particular theme.
You can, with the paid CSS upgrade change the CSS for a particular theme. CSS controls the look of a theme, but cannot be used to add or change functionality. It is about styling only.