Hi Ann,
I downloaded and installed a plain DITA-OT 4.2.2 installation, then ran the following command:
./dita-ot-4.2.2/bin/dita \
-i dita-ot-4.2.2/docsrc/userguide.ditamap \
-f html5 \
-Dargs.css=dita-ot-doc.css \
-Dargs.cssroot=`realpath dita-ot-4.2.2/doc/css` \
-Dargs.copycss=yes
and the output used the CSS as expected. Be sure to use args.cssroot to specify the absolute path to the directory that contains the CSS file. I'm using Linux, so I used realpath to resolve a relative path to an absolute path.
- Chris