First, you will have to use an FTP program and login in to the location of your Joomla site. You will have to use an FTP program that allows you to change permissions directly on the server, most do. Also, you will have to have your site hosted by an ISP that allows you to change file permissions. It's rare these days to have an ISP that will not allow you to change permissions.

If you want to change the permissions on a template, after logging in to the root of your server via FTP, go to "Templates" and then look for your default template, enter that directory and then look for "CSS". Look for the name of the file that needs to have its permissions changed. Highlight it then select "Info" or "Permissions" or use whatever method your FTP program uses for allowing you to view the permission setting on a particular file.

You want to make sure that "Owner Access" is set to on. This is giving permission for the file to be "Writable" by only the owner. DO NOT make any of the other owners writeable, the other two are "Group Access" and "Others" as this will create a security problem by allowing visitors to change the document.

Close the FTP Program. Go back to the Administration area of your Joomla install, check the CSS area and your file should now show as "Writable" in green.

The same technique can be used on any document that is showing as unwritable, regardless of what type of file it is. You will just have to find the location of the document on your server to make the change.

Please make sure you understand this procedure well before attempting to make any permissions changes. If you feel uncomfortable with this, contact your ISP for assistance.