You want to create a new “screendesign” package based on the “default” package’s “default” theme.

Copy app/design/base to app/design/screendesign

Create an empty folder app/design/frontend/screendesign and copy app/design/frontend/default/default into it

Create skin/frontend/screendesign, then copy the CSS and images folders and their files from /skin/frontend/default/default/ to skin/frontend/screendesign/default

Inside app/design/frontend and skin/frontend, copy the folder “default” to the folder “custom”; then rename custom/default to custom/screendesign in both places

Create app/design/frontend/default/screendesign