Since there is some need to add plugin's css style into redmine's page, I think it is a good place to add it in the plugin hook of view_layouts_base_html_head.

But I need to know the controller name and action name before doing this, because only a few pages need plugin's css code.

Example:
Add plugin's activity into redmine's activity page need an additional css code.

It's simple to implement, just modify two lines in base.rhtml:

ln17: <%= call_hook :view_layouts_base_html_head, :controller => @controller %>
ln64: <%= call_hook :view_layouts_base_body_bottom, :controller => @controller %>