Trouble editing theme

I’m a clueless newbie, so please expect my problem to be very obvious!

I’m having trouble changing my permissions such that I can edit the presentation of my blog. I changed the permissions to 666 on the index and layout files on my host, but to no avail. The theme editor still tells me that the files are not writable. Did I change the permissions on the wrong files?

I’d appreciate any help. Thanks.