As we have told in the
NOTE: Please use the child theme style.css or custom css plugin to write the css.
If you are using child theme then you will need to follow steps to keep the css;
Go to Dashboard => Appearance => Editor => Then keep the code in style.css and save it.
And if you are using simple custom css plugin then you need to follow following steps to keep the css;
Go to Dashboard => Appearance => custom css=> Then keep the code inside the custom css box and save it.
We hope you understand it.