mb2 class and the inline style only affect one element. In that regard, yes you nee to update each and every instance of
mb2 if you make a design change.
Thanks for the kind words Pascal Aubort. And sorry for the late reply!
Extending classes is fine, there is no right or wrong. You could indeed do with Sass something similar to what TailwindCSS does with PostCSS and the @apply directive: https://tailwindcss.com/docs/extracting-components
So, the key to understand is that utility-first CSS does not prevent you from writing regular CSS.
Actually, it makes it very convenient to add custom CSS, because all the utility classes have very low specificity. That means that when you write custom CSS, they never have to fight for specificity to override the utility…