Just updated to the new WordPress release, and the Theme Editor doesn't work within Opera 10. I know Opera 10 isn't released as a stable release yet, but it's running perfectly fine for the most part. It's just most things are changing on websites, and Opera doesn't cope with the changes. (One website in particular was Facebook, which was steadily fixed by Opera Dev. due to a change on Facebook's side. Though it isn't totally fixed yet.)
Just a heads up for the dev team. We'll see if Opera can modify itself to the change.