How to overwrite default styling of ERP5 site without messing with core code

There are sometimes cases when one wants to modify the look of ERP5 site - either to please the client, or to add some dynamic behaviour to the style (e.g. differentiate something by url or user name - very useful for testing). The style sits in erp5.css in erp5_xhtml_style skin; though, for obvious reasons, it is not a good idea to overwrite it or modify it.

The solution to create a Python script named "erp5.css", like this:

context.REQUEST.RESPONSE.setHeader('Content-type', 'text/css')
css = getattr(context.portal_skins.erp5_xhtml_style, "erp5.css")(portal_url=context.getPortalObject().absolute_url)
css += getattr(context, "my_style.css")()
if whatever_condition:
  css += getattr(context, "conditional_style.css")()
return css

The script covers the original erp5.css, and retrieves its contents by acquiring it directly from the erp5_xhtml_skin, so that we do not have circular import. Then it can import other sheets, or do any postprocessing imagineable. Sets the appropriate header and returns the final stylesheet. That's all.