What a style guide is actually for
Not for consistency. For making decisions faster, and for being able to say no.
People say a style guide keeps things consistent. That is true, and it is the least interesting thing about it.
It makes decisions cheaper
Every project contains hundreds of tiny decisions: which blue, how much space, where the line breaks. A guide answers them once. After that, the only decisions left are the ones that matter.
It lets you say no
The most useful page in my own guide is the one that lists what I do not do: no gradients, no shadows, no all-caps labels. When a request breaks a rule, I can point at the page instead of having an opinion.
It should be short
A guide nobody reads is decoration. Mine is a handful of pages: colour, type, space, and a page of rules. Anything that needs a paragraph to explain is probably two rules.