Another best SMACSS practice I find useful is the concept of the container or slot.
How to better organize your CSS architecture with OOCSS, BEM, & SMACSS
Claudio Mendonca
7334

The main takeaways I got from SMACSS are themes and state classes. So light/dark theme or summer/winter theme. State classes such as is-open for an accordion or is-dirty for a form control.