Can’t edit my stylesheet

To edit files in the theme editor – any file edited must have a chmod of 666. This isn’t really secure, so I would suggest downloading the file, edit it in a basic editor and re-upload.

I’ve written a plugin to help people change their CSS even if they can’t change the file permissions:

https://www.4-14.org.uk/wordpress-plugins/style-tweaker

For future reference on the subject of making your files writable in WordPress, see Changing File Permissions.