Since WordPress 4.7 it is possible to add a custom CSS directly in a theme customizer: just navigate to Appearance → Customize → Additional CSS.
This custom CSS will be specific to the theme you are using (it is basically the same approach as using a child theme).
TIP: Watch a video on customizing a theme CSS.
Using a child theme
If you are using a child theme, just place your custom CSS into your child theme's style.css file. Using a child theme is recommended solution as you can make any theme modification there (including a PHP modifications) and manage all of them in one place.
Using a plugin
You can also use a plugin to add a custom CSS to your WordPress website:
What is CSS?
You can learn more on CSS at http://www.w3schools.com/css/
To find out what CSS code you need to apply to modify a specific theme elements use a code inspector tool in your browser (you can also watch a video on using Chrome Developer Tools on WordPress.tv).