I'm trying to add a custom field to the built in Page content type. I've found lots of tutorials on how to do this for a single page, but I want to do this for all pages in my theme.
Please point me in the right direction.
If you want to do that yourself, take a look at function
add_meta_box (don't forget the examples). That page has almost everything you need to create a custom meta box for a certain post type.
If you want to simplify your work, you might want to use an utility like Meta Box plugin. This plugin can help you register your custom meta box with many built-in fields.