I'm not entirely sure if I'm reading the syntax right, but when a function is defining a variable string, this is effectively taking all the globals (ie, font size, font base, and so on) and compiling them into a local variable (ie $page_font) and using that in the CSS instead.

Presuming that's correct, then this means the logic is that you have property, set value, create tossed-salad of globals using values as set in this instance and output local variables. Thus if you reset a property (as for a child theme), this tossed-salad step rejuggles the property values into a new local variable. Yes? No?

Hm. What is the reasoning (if any) behind not just taking the global straight up, that is, instead of:
var string page_background = generate_background_css ... etc etc
which shows up again as:
body { $page_background ... etc etc
why not skip the combination step and just do:
body { background: $*color_page_whatever url($*url_page_whatever) ... etc etc

Or is the combine-global-with-this-value a required step to be able to do children styles?

(Babe, soon as you saw my name on here you probably knew what was coming, eh. I can't make heads nor tails of any language until I can grasp the grammar. Aka, "but why can't I say ain't?" !)