I really need help with themes and setting css items in program

[Damon Register <damon w register lmco com>]

I have been using gtkmm 2.22 for many years but would really like to move
on to Gtk3.  I have made a few attempts over the years but the thing that
always stops me is dealing with themes in the Gtk3 world.  A simple
hello world program produces ugly oversized widgets.  Over the last few
days I have put some effort into learning gtk3 theme tampering and have
made quite a bit of progress.

Perhaps the biggest roadblock for me is that most (if not all) of the
Gtk3 themes override widget size and usually make them bigger than what
what I specified in my Glade file.  Is there a way to stop that behavior
on a global level or will I just have to tamper with theme settings for
things on a more individual level?

From a post to this list 4 years ago I have learned that the win32 theme
is built in now.  The answer then was to tamper with gtk-3.0/gtk.css
but I don't find this at all in my msys2 setup on Windows 10.  I do find
C:\msys64\mingw64\share\themes and there is a Default\gtk-3.0 but would
be Adwaita, wouldn't it?  Is there a css file for win32 or is it built
into the code?

If there is no css file for the default or win32, how can I do this within
the program?  I am slowly finding info about working with css and have
learned a little tampering but I find nothing about doing this in the
program.

I am really stuck here and would greatly appreciate any help with this.
Thanks.

Damon Register