gtk_progress_bar thickness related to theme?

[Mark Cianfaglione <m cianfaglione valydate com>]

I've developed an application using GTK3 and the theme seems to be more
important on the look and feel than with GTK2.

In my GTK3 application (for Windows 64 using MinGW-w64) the
gtk_progress_bar is thin. Is the thickness of the progress bar related
to the theme? (I only have Adwaita) 

How would you change the theme for a compiled app that will be
distributed on Windows? (Without having to install MSYS2 or MinGW=W64)

Mark