Now-a-days many Themes provide Custom CSS as the theme’s inbuilt functionality. It is in-fact a good practice to use Custom CSS as it is in the Theme options panel, so it would not lost in updates. You can override the theme’s CSS with your own as per your requirement (for changing the style, color etc as per your requirement) in the Custom CSS field. And the most important thing is that it will not lost even after you update your theme.
But the question is that what should you do if your theme does not prove inbuilt Custom CSS? Don’t worry. Still you can use Custom CSS in your theme. There are 2 ways you can use Custom CSS in your theme without touching the theme’s style.css.
1. By using a Child Theme.
You can create a Child Theme of your theme and add use the Custom CSS you want to add in the Child Theme’s style.css to achieve your requirement.