If you have a style.css file in your theme’s directory. It would be in Appearance -> Editor.
Don’t edit that file directly. You should create a child theme first and edit that child theme. Once you’ve created a child theme, you should upload the child styles.css file to your child theme’s directory and then you can activate it from the dashboard. From now on, any changes you need to make should be done from the dashboard -> Appearance -> Editor. Then activate the childe theme by going to Apperances->Themes. You can insert the code under the base code inside the child theme's style.css file.