I am trying to modify the border around thumbnail slides in a "lens" in slidedeck 2. Instead of editing the source files of the plugin, I'd like to add a shortcode into the post. I am unsure where to add the function shortcode (in the functions.php file, or in the shortcode.php file, or somewhere else?) and how to direct it to a custom.css file to override the plugin's css. I have never written a shortcode function before and am a php beginner. You can see an example of the thumbnails here: pmotzkinaia.com . I want to remove the border of the thumbnail and right justify instead of center justify.
Any help or links to similar problems would be greatly appreciated. Thanks!
tl;dr: How to write a shortcode function to override a plugin's css.