adding CSS to a widget plugin

I have installed a widget plugin that presents category posts in the sidebar. I would like to add css to it, but I don’t understand the php well enough to know how to do this.

How can I get it to either a) inherit current styles in the page body content, and/or b) apply attributes just to the widget (which would come from the theme stylesheet?

I do know css, it’s just a matter of how to incorporate it into the php of a widget.

Thank you!