Where To Edit CSS to Change Sidebar Font-WordPress 2015 Theme

I am using the WordPress 2015 theme and want to increase the font size of the text that is on the sidebar of my website (saint-albans.org). I’ve never edited the stylesheet of a theme before, so I need to know what part of the CSS controls the font size of the sidebar menu and any suggestions regarding changes/additions to make to the stylesheet. Thanks for any suggestions!