All of the available documentation for HTML Editor is on the HTML Editor Support page:
html-editor/
The getting started guide has a little info an using the styles tab, as does the help documentation:
styling.html
On the styling tab, click the plus button to add your style sheet.
then click the pencil icon next to "Modify the options listed in the Styles menu of the HTML Editor Toolbar" Then Click the Import button and select the styles that should be imported.