You will need to add styling rules to the end of either the plugin stylesheet or the theme stylesheet. Depending on what theme you’re using you may have a theme option to add some custom styling rules.
Note if you put it in the plugin stylesheet you will loss the custom style next time you update the plugin. The same would hold true for the theme style sheet if you were to update the theme.
The following style rule will make the link color black and when you hover over the link it would become gray.