Update custom.css to offer a Material Design theme

[jason-fox]

The custom.css used to be a random Bootswatch mashup.
This is an updated theme is based on a Material Design 3 theme and therefore
is complementary to other Bootstrap/Material initiatives such as: https://github.com/mdbootstrap/mdb-ui-kit

dita -i ../plugins/dita-bootstrap.specialization/sample/document.ditamap -f html5-bootstrap  \
     --args.hdr ../dita-bootstrap/.github/dita-ot/header.xml  \
     --bootstrap.theme=none \
     --args.css=custom.css \
     --args.copycss=yes \
     --args.csspath=css \
     --args.cssroot=/add/path/to/plugins/dita-bootstrap/css \
     --nav-toc=collapsible \
     --css.theme.switcher=no

[jason-fox]

There possibly needs to be an update to the documentation somewhere too, but I'm not sure where this would best fit in.