Blogging on my education was not something I planned on doing; but, this tip is too important not to pass along to someone learning CSS. It is not covered in any of the video courses seen recently. Do NOT make changes to the theme style sheet directly. Save your changes to a custom style sheet instead.
All video course material seen recently, whether Mac or PC, has been centered on making changes directly to the theme style sheet and viewing those changes immediately.
In WordPress this would be done using Appearance|Editor and saved on exit. At this point you are a theme developer responsible for all future updates, or your changes will be over-ridden, left for you to re-make them when the theme is updated.
To avoid this scenario, navigate to Appearance|Edit CSS in WordPress.
Learn to use this piece. Changes that you view in the theme style sheet are copied here to a custom cascading style sheet. With CSS, last one wins. Your changes will dominate in execution order and stay in place through updates. Made in the theme style sheet and they are lost.
It will take a little bit of experimenting; but, it isn’t hard to grasp. In the theme style sheet, you grab a minimum amount of code to start, and you make your changes in the custom style sheet. Changes are similarly viewed immediately… just stored in a separate file.