Given that you are the webhost:
If the server is a dedicated server, or you’re in any situation where you’re the only user of the server in question, then this is a GOOD thing! It means your setup is more secure this way.
It basically means that somebody who hacks into your site through the webserver “door” will have a limited set of permissions, and will be unable to edit files. Which is exactly what you want to happen.
If this is a “shared” server, where you’re hosting more than one site and have more than one “user”, then you need to change the webserver to run using a method known as “SetUID”. There’s many ways to do this, suPHP, FastCGI in a setuid wrapper, etc. This makes the PHP process which is running the site run as the user who owns the PHP files, providing increased intra-user security.
Now, you may think, “why would running as the owner of the files be more secure?” Answer: Different circumstances. When more than one user has a web directory on the site, all being served by the same webserver, then the webserver user has access to all those users files (read-only, sometimes more). But each user should be locked down to see their own files. So in the event of a compromise, you want the hacker to have the rights only of one of those users, not of the webserver user.
So, basically, if you really want, you can change the ownership of the theme files to be those of the webserver user (or rather, whatever user the PHP process is running as), and thus get access to them in the WP editor. But, unless you’re setup for a shared server environment and using setuid wrappers to be more secure in that fashion, then you really don’t want that.