It appears that for EPUB files, -css doesn't work. And I got this to work by using --epub-stylesheet=style01.css:
c:\chuck\ebook\pandoc\pandoc -f markdown_mmd+backtick_code_blocks+yaml_metadata_block -t epub -o %outfile% --toc --epub-stylesheet=style01.css 00intro.md 01.md
Someone should probably put a note under the -css option that it doesn't work for EPUB books and the note should say "use --epub-stylesheet=file.css instead".