An external CSS file, when you edit it, if it was already linked to your webpage file, will edit the webpage according to what you have specified in the CSS file. So, it is easy to edit your webpage background and text using the external CSS file. However, an external CSS file will be used throughout the entire webpage.
If you do not want that, then you will have to do an internal CSS file. The internal CSS file will be more troublesome to edit though, as you will have to scroll through your entire HTML coding to edit it. However, you can have more designs your webpage using the internal CSS.
Combining HTML with CSS, your webpage will look much better, compared to if you only use HTML to do up your whole website.
