There are two methods of containment CSS in HTML.
You can attach CSS document to HTML document. In this case, style sheet
stored in separate file and you can use one style sheet for many HTML documents.
JustStyle adds <link> tag into HTML document only.
Use Tool -> Attach to HTML
file... choice in the Menu Bar for attaching style sheets to HTML
documents.
Also you can insert style sheet into HTML document. In
this way style sheet stored in HTML file is applied only for elements in
the HTML file, where style sheet stored. Others HTML documents can't have
an access to style sheet stored in HTML document.
Use Tool -> Insert
into HTML file...
choice in the Menu Bar for attaching
style sheets to HTML documents.