WhiteSur-gtk-theme/src/sass/gnome-shell/common/_buttons.scss

/* Buttons */

.button {
  min-height: $medium_size;
  padding: 0 16px;
  border-radius: $bt_radius;

  &, .popup-menu & {
    @include button(flat-normal);
    &:hover { @include button(flat-hover); }
    &:active { @include button(flat-active); }
    &:insensitive { @include button(flat-insensitive); }
    &:focus { @include button(flat-focus); }
    border: none;
  }
}