If you want to change small CSS bits of your theme and don’t feel like a Child Theme is needed, then you can do so as long as you paste the CSS in the right place.
The wrong place would be to edit style.css/responsive.css. The right place is to go to Customize > Additional CSS. Here you can add your custom CSS and it will be safe when you update the parent theme.
Most of the time however, a Child Theme is the best option in the long run – but this is a valid option when just doing CSS edits to your theme.
Working with CSS
If you want to learn more about CSS editing, my suggestion would be to download Firefox. Once installed, go to your website and Right Click > Inspect Element. Here you can modify the CSS code live and see how your changes affect the page directly. Once you are happy with your changes, simply add them to your custom CSS place, whether that is in your Child Theme’s style.css or the Additional CSS field in the live customizer.
There are a ton of guides out there how to use the built-in Firefox editor, I would suggest using YouTube to learn the basics. It is a must-have tool if you plan to work with CSS.