~/.config/gtk-3.0/gtk.css
.window-frame, .window-frame:backdrop {
box-shadow: 0 0 0 black;
border-style: none;
margin: 0;
border-radius: 0;
}
.titlebar {
border-radius: 0;
}
.window-frame.csd.popup {
box-shadow: 0 1px 2px rgba(0, 0, 0, 0.2), 0 0 0 1px rgba(0, 0, 0, 0.13);
}
.header-bar {
background-image: none;
background-color: #ededed;
box-shadow: none;
}
/* You may want to use this if you don't like the double title.
GtkLabel.title {
opacity: 0;
}*/