Hello,
I would suggest you edit the skin.css file before uploading it to your live DNN installation. If you have a localhost installation, it's a much easier method to create the skin at localhost level and then upload the skin when it is complete.
If you need to make CSS changes from your DNN portal, the only option you have is to go to admin / site settings and in there you will find the style sheet that you can edit.
This will edit the portal.css file
The portal.css file overrides any settings in the default.css and skin.css file.
If you are creating skins, I would recommend you keep all of your CSS within the skin.css file for simplicity and ease of use when you come to make any changes in the future. - Sometimes mistakes can also be made and you could lose the data in your portal.css file.
This article may explain a few things for you:
http://www.dnncreative.com/Tutorial...fault.aspxas they will make it much easier and quicker for you to pick up how to skin a DNN site.
Thanks,