You can add your own stylesheet for users to use within Interspire Website Publisher quite easily.
Firstly, open /admin/includes/classes/class.editor.php.
On or about line 141 you should see this:
// Setup the toolbars
$rows = array (
"Fullscreen,-,Undo,Redo,-,Paste,Spellcheck,Clearcode,-,Bold,Underline,Italic,-,OrderedList,UnorderedList,-,JustifyLeft,JustifyCenter,JustifyRight,JustifyFull",
"Fontname,Fontsize,-,ClearFont,Fontcolor,Highlight,-,,CreateLink,Image,Flash,Media,-,Help",
);
Replace it with:
// Setup the toolbars
$rows = array (
"Fullscreen,-,Undo,Redo,-,Paste,Spellcheck,Clearcode,-,Bold,Underline,Italic,-,OrderedList,UnorderedList,-,JustifyLeft,JustifyCenter,JustifyRight,JustifyFull",
"Fontname,Fontsize,Styles,-,ClearFont,Fontcolor,Highlight,-,,CreateLink,Image,Flash,Media,-,Help",
);
$myDE->SetSnippetStylesheet("/path/to/your/stylesheet.css");
The Bold text is what needs to be added.
Make sure to modify the path to the stylesheet to match your server path.
