[Plugin: WordPress Related Posts] Customize/Style WP Related Posts Plugin

I like that the plugin shows up under the content. but how can i customize the look of it. the header is too big and i’d like to give it a background, and make the overall font of the links smaller as well. is this a simple CSS fix? If so, how do I do it?

Thanks..

https://www.ads-software.com/extend/plugins/wordpress-23-related-posts-plugin/