Anomalum I don't think there's any need for a long explanation on this, but in short; weβre not writing CSS code for a project here. Custom CSS actually allows us to do the very things that are considered bad practice in the article you shared. I'm not sure how long you've been working with CSS, but I was using CSS code even before HTML5 and CSS3.
In short, "custom CSS" is exactly the place where you would need to use !important. However, if you ever plan to write an extension or theme for Flarum, you should avoid using it there.