How to add custom CSS to my WordPress theme?

Sometimes your theme may require CSS customisations to work properly. Adding CSS to your theme can be done in a few different ways: 

1) Use Customizer. Go to your WP Admin page, then go to Themes > Customize. In the customiser, you should have a sidebar displaying many options. Depending on your theme, look for one called "Additional CSS" and open this. You are then given a textarea to paste your custom CSS in to.

2) Add the custom CSS to your theme / child themes style.css file. Add your custom CSS to your style.css and then upload to your server. Be careful that if you add the CSS to a parent theme (i.e. a free theme downloaded from somewhere) your customisations may disappear when this theme updates. If you use a child theme, you can add customisations and still safely update the parent theme.