Need to add some CSS to Flexi Widget

I modified the flexi-pages-widget.php to display the selected pages thumbnail and title using the widget in one of my template’s sidebar postions. But now I’d like to add some styling to the widget (ie. padding, img size, etc.). Can someone please shed a little light on how to style the widget? There is no included stylesheet for the Flexi Pages widget. And the only thing that I can seem to find that refers to the styling says that you have to add .flexipages_widget" to the theme’s style.css in order to add custom styling. HOWEVER, I cannot seem to get this to work at all. For instance, if I add .flexipages_widget a { color: #0f0; } as the FAQ suggests, it does NOT change anything for the widget’s output on the site.

I am currently working on the website located here: https://s119736768.onlinehome.us

I would like to have the widget display the pages thumbnails in a 3 column, vertical format. Is there a way to add a stylesheet to the widget instead of using the theme’s stylesheet? If not, what CSS should I be using in the theme’s style.css in order to define the styles that I need? PLEASE help!

https://www.ads-software.com/extend/plugins/flexi-pages-widget/