Since WordPress 4.7, you can now easily add custom CSS to your website from within the WordPress Customizer.
If you go to Customize -> Additional CSS you will see a code area for adding in your own custom CSS to further customize your site.
There might be a CSS comment in there when you first open the Additional CSS section, you can simply remove the CSS comment and start adding your own custom CSS.
Read more here: https://codex.wordpress.org/CSS
Add CSS via a plugin:
Another option is to use a plugin to add your custom CSS.
One of the most popular plugins for custom CSS is Simple Custom CSS.
Add CSS via a child theme:
You can create a child theme and then add your own CSS into the child theme style sheet and this will override your parent theme styles.
Or you can use one of our child themes as a starting point to change theme CSS and php if you need to.
If you’re simply adding CSS then we recommend one of the options above. Child themes are more for editing php templates.