You can change or add custom CSS code using the plugin. You can find the Custom CSS module in Jetpack plugin WordPress. In this case, after activating the Jetpack plugin and module custom CSS, you will find the option Edit CSS in the Appearance section.

Instead, you can use other plugin Simple Custom CSS.

After you have installed and activated the plugin, you can enter your CSS code on Appearance > Custom CSS.

Learn More:
How to Customize a WordPress Theme