So ...

As I said, the standard theme customizer will allow you to easily add your own CSS rules from the WordPress admin area.

If you go to Appearance => Customize, you can launch the customizer for your active theme:

If you go to Appearance => Themes, you'll see a "Customize" button that will allow you to launch the customizer for your active theme, and a "Live Preview" button that will allow you to launch the customizer for another inactive theme:

And each theme customizer has a certain menu item that will allow you to add your own CSS rules. Usually, it's called "Additional CSS" ...

... and is nothing else but a simple entry field where can add your custom CSS code:

As you can see, it's super-easy and you can see the changes with a live preview instantly ...

BUT! Any custom CSS that you add using the theme customizer is only available with that particular theme!

If you change your theme you'll have to repeat the same process ... And the real problem: any new Wordpress and/or theme update (even the automatic updates) can - and usually will - remove your custom CSS rules!

This is why you need a more reliable plugin-based solution ...