You must create a child theme, and write in it the added CSS.
You need to use the Chrome Dev tools in order to inspect the areas thar are needed to be modified.
How do I use dev tools in Chrome?
From the Chrome menu: Open the Chrome menu and go to “More Tools” > “Developer Tools.” Finally, you can right-click (Windows) or Ctrl-click (Mac) anything on a web page and select “Inspect Element” to open Developer Tools. The Developer Tools panel will open in whatever web page you're on.
