Just a few changes? Keep it simple.
In the Customizer
In WordPress 4.7+, use the Additional CSS section in the Customizer.
Note: Any custom CSS added in the Customizer is only available for that particular theme. To use your custom CSS in other themes, simply copy and paste it the same way for each theme.
One big advantage to using the Customizer is that you will see your changes right away so you can easily dial in your CSS without saving files and flipping between browser windows.
In your theme
Some themes offer a custom CSS section too. Like the native WordPress option above, your CSS will only be available when that theme is active.
With a plugin
Try a plugin like Simple CSS for theme-agnostic changes and more room to type.
Lots of changes? Make the pro move.
Create a child theme and place your custom CSS in its stylesheet.
Child themes require a little more work up front but offer many benefits down the road, including:
- the stylesheet can be cached, improving your site's speed
- they are safe from being overwritten by theme updates
- they make further customization easier; for example, custom post templates
What NOT to do
Modify the plugin stylesheets. Your changes will be overwritten in the next plugin update, which are frequent.
Modify the theme stylesheet if you are using a free theme from wordpress.org or a premium theme. Your changes will be overwritten in the next theme update.