I highly recommend that you DON'T use SGC coding in your stylesheet.
It will cause confusion
AND
If you happen to do something in Sigil that creates a Local version of the same code

,
it will take precedence.
Believe in Murphy..
Avoid doing things in ways that help errors happen

Pick your own names for selectors or at least modify them in ways that could never be generated by Sigil (or Calibre), just to avoid confusion.