About flocss

I am using flocss, but
Similar to the site-specific css (margin, padding, width, color) OOCSS structure?
and
css (not very honest, but is it display:block, etc.) that is common to all sites
I think both are in one element, but
How do you separate them?

For example, .c-btn.p-btn-skin or p-skin?
However, isn't it difficult to manage with c- and p- even though they are the same elements?

.c-btn.c-btn-skin is also available, but Mr. Tani said it is better not to wear skin on c-, so
It's easy to manage, but I wonder what it is like.

How on earth are professionals doing?
I thought it would be good to reuse it, but there are useless classes and css left.
There are many problems like which class to delete.