To override any CSS, you need a new style that is more specific.
CSS styles are applied in the order that they are written. If you have two identical selectors, the one that is read last will override any duplicate properties in the previous style. But, if one use more specific (i.e. more restrictive) than the other, then those properties will take precedence.
In your case, if you simply find the selector that is being used for the style you want to override, you can copy it into a CSS file that is loaded after the plug-in's CSS file, and then override whatever you want.
For more information about specificity, see this article on CSS-Tricks.