Go to your plugins page and delete the plugin via the admin panel. I think you do this by clicking the checkbox and then somewhere else on the page is a select box which allows you to delete it … or at least it works something like that.
By deleting it that way, it will trigger the uninstall script built into the plugin which should wipe out the original CSS along with any other settings you had made in the plugin.
Then re-upload the plugin and activate. It should work as it did when you first installed it then.