@Paul Lefebvre has posted a great BLOG and some examples for controlling themes in your app for Linux and GTK3.
GTK3 Can Be a Pain in the Themes
This looks like a very good step forward to resolving Linux GTK3 themes and keeping our layouts sane.
Well, if they cant solve the windows UI problems with native windows code, maybe they ccould implement GTK
Here’s a simple examle - the first shot is CentOS Adwaita with default, non-managed theme control:
This is with the app-included theme:
There’s still some further adjustments in the theme, but you can see the buttons now look correct at the default size.