I don't think I emphasized this enough, but I only observe this under these circumstances:
My Host & Admin settings for Host/Edit skins and Portal/Edit skins are all MinimalExtropy -index and Minimal Extropy - title_grey.
I change one page's Page Skin to my own skin (skin mikevdm, which has skin.ss and main.css in the skin folder).
Now I view the page, all is well (my skin mikevdm is used).
Then I click Settings in ControlPanel.
The resulting Page Settings page uses the MinimalExtropy skin (as I would expect based on my use of 4.9.x and my current settings).
But, instead of pulling in "/Portals/_default/Skins/MinimalExtropy/index.css", it brings in my skin's "/Portals/_default/Skins/mikevdm/main.css".
Again, I have tried this on a clean 5.0.0 install without any modifications other that adding the skin.