Capp Oh, it's really quite easy.
I'll try to break it down into five simple steps with a few straight forward examples.
Step №1:
• Access your [Administration] area.
Step Nº2:
• Go into the [Appearance] interface.
Step Nº3:
• Open the [Custom Styles] section.

Step Nº4:
• Add your preferred [CSS] code.
(Here are two examples)
(i) Adding Subarist's code:

(ii) Alternatively, adding Anomalum's code:

Step №5:
• Hit [Save], then check the front end results, and if necessary, further modify the code to your liking. Job done.