dotfiles/config/gtk3/gtk.css

/* FORCE EVERYTHING SQUARE - GTK3 */
* {
    border-radius: 0px;
}

window,
decoration,
.window-frame,
.window,
.background,
.titlebar,
headerbar,
dialog,
popover,
menu,
tooltip,
button,
entry {
    border-radius: 0px;
}