Our CSS is a mess and needs to improve. Let's have some rules for that.
Our habit to style elements makes it easy to swap elements without affecting style.
We need buttons everywhere and they are more or less the same. So use a single class button and add as many modifiers (additional classes) as nesesarry for all small exceptions.
Need a paragraph with a special margin and a special color? Make two modifiers; one for the margin and one for the color.
Nesting leads to large selectors, especially if high up the selector change there is a comma in a selector.
If at all posible and reasonable, define an additional class. If possible and reasonable, make the class generic; if you need to add some extra margin to some object, make that a generic extra margin class, don't apply it to just one single element. This makes the class much more reusable.