i'm a new user of wordpress. i've previously used blogdrive. i downloaded a few themes in css format frm some websites. why cant i use it in wordpress? MUST i purchase the custom CSS upgrade in order to use it?
Premium (and free) WP CSS themes are for WP sites on your own host. If you are trying to use them on wordpress.com, you will need to buy the css upgrade. (Probably a few other upgrades as well.)
Then you will not be able to ADD a theme. You will only be able to use the themes provided by them, but you will have the option of buying credits and customizing the CSS on a theme.
Would suggest you to get of from wordpress.com blog, and make it with your own domain. Gives you huge freedom to play with possibilities of wordpress platform.
I agree with Oranges, it's much better if you host your own WP blog on your own hosting account. You have much more flexibility and can add all kinds of customizations and themes. There are lots of beautiful free and paid WP themes.
There are plenty of open source beautiful themes as good as paid ones. Check out www.top-best-themes.com