Adding new style sheets and Javascript scripts in Plone 2.0 was problematic, because all such resources were hard-coded into page templates. In Plone 2.1, ResourceRegistries provides the tools portal_css and portal_javascripts which can be used to register CSS style sheets or Javascript files, respectively. Resources can be loaded conditionally, determined by a TALES expression, and are easily managed using the tools' APIs.

RichDocument registers a custom styles heet with 'portal_css'[1] in its 'Install.py':

    portal_css = getToolByName(self, 'portal_css')
    portal_css.manage_addStylesheet(id = 'richdocument.css',
                                    expression = 'python:object.getTypeInfo().getId() == "RichDocument"',
                                    media = 'all',
                                    title = 'RichDocument styles',
                                    enabled = True)

Notice how the expression variable is set to a TAL condition that is evaluated to determine if and when the style sheet should be included.

Use the DocFinderTab on portal_css and portal_javascripts to find out what else these tools can do.

If you are interested in creating a visual style/theme for Plone, the DIYPloneStyle skeleton product[2] should get you started.

[1] Note that it is not strictly necessary to use portal_css in RichDocument's case, because the stylesheet is only used in the richdocument_view* templates, which could have used css_slot to define them as normal. However, that would not have made as good a demonstration. ;-)

[2] This supersedes the older SimplePloneStyle product, which may still be useful as an example.