Hi Jason, I am not sure how other people go about doing it, but it is a really great question.
I usually end up creating an extra page that is a sea of empty divs with just those global properties.. I park copies of my global abstracted properties I am going to apply to multiple objects. Then when I use Webflow's auto clean up, I don't accidentally delete something I went to great pains to create, if it is not currently used.
To start, I think about class structure as I go and create base placeholders of what properties I want to make global. Some of it happens later when I decide that it would be an advantage to single out a property, so for me, jumping back and forth to the global abstracted junk pile of divs is not really as tedious as it sounds.
It is still a bit more challenging than it would be directly in a code editor, but webflow affords a zillion other advantages. I frequently export my code and look over the CSS to see if I have elements of those classes that have unnecessary duplicate properties or weird structure. Then I can go back into webflow select my global class from the dropdown and easily clean them up or part them out using the blue and orange property (inheritance) highlighting that shows up in the designer.
Would love to hear other peoples approach - I am sure they will have better advice This would be a good subject for a tutorial from one of the experts as it has been one of the bigger challenges for me to sort out in practice.