Don't Have Permission to Edit Theme

I just installed wordpress on my local machine, and immediately went to edit the theme, but when I went to save my changes, it said the file wasn’t writable.

Obviously, the file is writable, because I can change it. The problem is that wordpress doesn’t have permission to write the file. How do I run wordpress using my permissions to edit the file. I don’t want to go through and change every file to be able to be edited by everybody.

I’m running using php and MySQL, I didn’t install MAMP, because I want to have multiple sites, and I don’t think you can do that with MAMP.

Can anyone help me out and tell me how to do this?