blockquote>I would also like to design a custom theme, but I do not know CSS (although I did purchase the custom design upgrade). I would be open to purchasing a premium theme from another site, but I don't know how to go about that while still keeping all of my content.
It's important to know that there in no FTP access to free hosted WordPress.COM blogs like the one linked to your username and we cannot install individual plugins or third-party themes into them. Those found on the internet are for WordPress.ORG installs which run on different software.
See also > WordPress.com vs. WordPress.org: The Differences > http://support.wordpress.com/com-vs-org/
On free hosted wordpress.COM blogs we can only use the themes which have been coded to run on this multi-user software and made available to us here. > Appearance > Themes and no others. This is a multiuser blogging platform. All blogs wearing the same theme are using the same underlying template and we cannot access the underlying files and edit templates or themes. http://en.support.wordpress.com/themes/editing-themes/
If you have CSS editing experience the upgrade will allow you to stylize the appearance of themes you find here > Appearance > Themes, but will not allow you to change the functionality, by editing the underlying template. It does not allow you to remove footer links to the theme designer or to WordPress.com. Also note that CSS is theme specific, so you cannot use the CSS style sheets from other themes with the themes at WordPress.COM. http://en.support.wordpress.com/custom-design/custom-css/
Lastly, please note that there is no upgrade you can purchase from WordPress.com that changes any of the foregoing. All blogs being free hosted by WordPress.com are subject to the same ToS and the same [policies not matter how many upgrades one purchases.