Try setting the "GTK_DEBUG" environment variable to "interactive",
this will open an extra window that will allow you change the css on
the fly.

i had a hunch that this might only work under administrator account.
correct. so i played with the interactive debugger...

It currently uses the default theme "adwaita", there is an win32 theme
that comes with the installer, but that also looks out of place at least
on windows 10.

switching back and forth between 3.18 and 3.20 i found out that i was
also using theme "adwaita" under 3.18. i kinda liked that design, sure
it is a bit different from standard windows but i thought it looked good.

apparently 3.20 comes with a reworked version of "adwaita" which is
unacceptable to me. i find the sharp bottom corners extremely ugly.
also, the whole design looks completely wrong on windows. (and there is
the problem of the missing app icon in the upper left corner)

i checked out the win32 theme but it is from the stone age and also not
acceptable.

so i need a different theme...

can this be added to the GTK library as is and how? or can i only do
this via MSYS2? is there a way to get the adwaita3.18 theme running on
3.20? or am i stuck with the themes as is?

and again the question, how can i make a new/different theme the default
for all GTK apps?

The css may have changed in the new release. i couldn't find GtkNotebook
in the default css at least.

unfortunately, i could not identify the source of the background color
problem using the interactive debugger. using GTK3.18 i see the
GtkNotebook "background-color" set to my gray value as intended, using
GTK3.20 the GtkNotebook "background-color" value is set to black, even
though it is displayed as white and i asked it to be gray...

also there is still the problem of the app icon missing.

thanks
/det