A thought on the various shades of grey for borders...
In my opinion(R), the idea behind going from black to grey for border colours is so that the less important borders (eg boroughs and counties) are less visible than the more important ones (eg national borders). However, when placed over a backdrop of typical country colours, the mid-level borders, being grey, are hard to see against the country colours (which in theory average out as some kind of grey). However, the lowest-level borders, being near-white, are actually more visible than the mid-level borders. I don't think it makes for a good aesthetic when boroughs can be seen more clearly than counties.
I'd like to suggest that the border colours be reworked so that the lowest-level border colour be a mid-level shade of grey, rather than an off-white.
Suggested values:
███ 0,0,0 : Coastline
███ 4,4,4 : National borders
███ 17,17,17 : Pseudo-National borders (e.g., UK 'countries', Spanish 'kingdoms' and viceroys, princely states)
███ 50,50,50 : Internal border (level 2)
███ 100,100,100 : Internal border (level 3)
███ 150,150,150 : Internal border (level 4)
(Note: Consider these colour values a basis for further discussion. I'm not married to the specific shades here)