How to decrease margins and padding in Gtk Themes (especially Pop!_OS)

I have xubuntu 18.04, and I really like Pop!_OS theme for Xfce ( The minimalistic icons and live pastel colors )

But I really hate how it waste space. The icons and buttons has huge margin around it. I'm looking into /usr/share/themes/Pop/gtk-2.0 especially main.rc and trying to figure out how to minimize all padding, margins and borders so that is use space more efficiently. But I really don't understand the file structure and my blind modifications did not yield any useful results.

This is how Pop!_OS theme waste the space - the icons cannot be seen on standard 24-pixel Xfce panel. The to of application labels are cut.

This is how it looks with some normal GreyBird theme: