WhiteSur-gtk-theme/src/sass/gnome-shell/widgets-40-0/_screen-shield.scss

#unlockDialogNotifications {
  StButton#vhandle, StButton#hhandle {
    background-color: transparentize($bg_color, 0.7);
    &:hover, &:focus { background-color: transparentize($bg_color, 0.5); }
    @if $gnome_version == 'old' {
      &:active { background-color: transparentize($primary_color, 0.5); }
    } @else {
      &:active { background-color: st-transparentize($primary_color, 0.5); }
    }
  }
}