@Gianfranco -

The only thing is that I don’t see how you could override the original CSS within the plugin itself. For example, I think that WP-Polls generates a scripts right before the tag with the to pull the CSS file within the plugin.

If so,because of its position, the plugin CSS file will override my plugins.css file, correct?

I’d really would love to know how can one simply choose to setup a plugins.css file that overrised all the CSS files for all plugins.

Depending on how the plugin implements its style, it can be difficult. Most plugins that I have run into, however, implement their style in a way that can be easily over-ridden by the user. (I don't know how that's done, because I'm not a plugin developer. :) )

In my experience, the plugin developer will just say, "the plugin is wrapped in a container with the .mypluginstyle class" - in which case, you just throw .mypluginstpyle into your plugins.css file, and away you go.