It depends on the theme. For some themes, there is a "Custom CSS" field under Appearance > Theme Options (not all themes have custom theme options) where you would paste what you need. Otherwise, you'll want to go to Appearance > Editor and will likely want to edit the "Stylesheet" from there (style.css).
This thread might be helpful. Also, be sure to edit your theme as a child theme so you still have the main one (in case anything goes wrong). More info on that here:
Hope that helps!