broken themes

i d/led a new theme and it had classic and default so i replaced my existing classic and default with those. i found out afterward that the classic and default folders in that new theme were just the php files and now i’m getting a message in my admin page saying “style sheet missing.” how do i re-download these two themes “classic” and “default”? i can’t even find them anywhere to download them. i know these are stupid questions but i’m new at this, so be easy on me. thanks. erica