1) Follow this link :
https://www.dropbox.com/s/pztqxwn38q2jxg3/wmd.css?dl=0
and download (top right button on the page) the wmd.css file
2) Copy it into the "pagedown" folder, that is here :
<your q2a doc root>\qa-plugin\q2a-markdown-editor\pagedown\
(or : <your q2a doc root>\qa-plugin\q2a-markdown-editor-master\pagedown\
check the exact naming you've used for the markdown plugin's folder)
**replacing** your previous wmd.css file (if you want, make a backup of it before replacing it)
3) clear completely your browser cache
You should now have the feature