Custom styles can be added from Appearance Customize Screen.
Keep in mind that the CSS changes are tied in with your theme. This means that if you change to a new theme, your custom CSS styles will no longer be active. If you change back to your previous theme, they will once again be there.
To find CSS classes or ID’s to style use browser inspect tools. This is a basic Google Chrome tutorial however, all other browsers have very similar workflow, so this can serve as a good starting point: How to inspect a page.