Off the top of my head as I’m on mobile, the best tool in your toolkit is the developer console – F12 in most browsers. With it you’ll be able to click into the element(s) in question to see both their identifying class/id in the HTML, and the applied CSS in the styles pane. From here you’ll be able to see whatever colours are in use in order to test your desired changes. The name of the stylesheet it’s in will be given there too (handy if there are multiple stylesheets in play). Use the given class/id selectors to locate the element in your CSS and make your changes.