WordPress default theme css units

Is there a specific rule the default WordPress themes follow for CSS units?

I see em’s on some font sizes and rems on others and px on certain places.

Referred the following links but I couldn’t find the actual reference to the rule they use for their CSS units:
why does twenty eleven theme mix up the css measurement units?
Why do WordPress default themes specify fonts in so many places?

Edit: By rule, I meant, for eg: Always follow the unit system of – px for borders, em for spacing etc

I would suggest looking into the ways that the various units function. There would rarely be a case where you would use one unit exclusively. I generally use ems, rems, and %.

This article may help get you started: understanding and using rem units in css