You should read that page again The foreground option is a modifier to the colorset command.
So you’d need to attach that to a colorset command, and then use the colorset in your pager to see any effect.
incidentally, I fiddled with the test pager script a bit more.
Colorset 23 Transparent, fg black, bg darkgray
Style TestPager ParentalRelativity
KillModule FvwmPager TestPager
DestroyModuleConfig FvwmPager: TestPager
*TestPager: Colorset * 23
Module FvwmPager TestPager
That gives me a transparent pager and black borders.
Do you redefine colorset 30 later in your config?