theme: sync toggle-off-dark with gtk

- improve contrast

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/2157

data/theme/toggle-off-dark.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="46" height="22"><defs><linearGradient id="a"><stop offset="0" stop-color="#39393a"/><stop offset="1" stop-color="#302f30"/></linearGradient><linearGradient xlink:href="#a" id="b" x1="53" y1="294.429" x2="53" y2="309.804" gradientUnits="userSpaceOnUse" gradientTransform="translate(-42.76)"/></defs><g transform="translate(0 -291.18)" stroke-width="1.085"><rect style="marker:none" width="44.446" height="20.911" x=".625" y="291.715" rx="10.455" ry="10.073" fill="#323233" stroke="#272728"/><rect ry="10.455" rx="10.455" y="291.715" x=".543" height="20.911" width="21.143" style="marker:none" fill="url(#b)" stroke="#151515"/></g></svg>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="46" height="22"><defs><linearGradient id="a"><stop offset="0" stop-color="#39393a"/><stop offset="1" stop-color="#302f30"/></linearGradient><linearGradient xlink:href="#a" id="b" x1="53" y1="294.429" x2="53" y2="309.804" gradientUnits="userSpaceOnUse" gradientTransform="translate(-42.76)"/></defs><g transform="translate(0 -291.18)" stroke-width="1.085" stroke="#151515"><rect style="marker:none" width="44.446" height="20.911" x=".625" y="291.715" rx="10.455" ry="10.073" fill="#282828"/><rect ry="10.455" rx="10.455" y="291.715" x=".543" height="20.911" width="21.143" style="marker:none" fill="url(#b)"/></g></svg>