Inspect the element which style you want to change.
Right-click the element and in a drop-down menu select something like "Inspect DOM element" or "Inspect element with Firebug".
Many browsers can do that natively, I personally prefer to use Palemoon browser with Firebug Add-on (IMO it's a bit more convenient).
There you could see all the selectors for the element.
On usage of CSS selectors I recommend to read
code.tutsplus.com/tutorials/the-30-css-s...-memorize--net-16048