Popup CSS for dark mode.

In Firefox, this requires version 67 for support for
prefers-color-scheme media queries.
https://hacks.mozilla.org/2019/05/firefox-67-dark-mode-css-webrender/
To force Firefox into dark mode, set ui.systemUsesDarkTheme=1 (and
optionally browser.in-content.dark-mode=true, to put pages such as
about:addons into dark mode as well) in about:config. You can check if
it's working at https://bugzilla.mozilla.org/, which has its own
dark-mode styling. Note that this kind of dark mode is *independent* of
the "Dark" theme that can be selected in about:addons.

Chrome requires version 76 for prefers-color-scheme. You can force it by
running with the --force-dark-mode command-line option.

proxy/static/embed.css

@@ -121,3 +121,27 @@ input:checked + .slider:before {
   -ms-transform: translateX(13px);
   transform: translateX(13px);
 }
+
+/* Dark Mode */
+@media (prefers-color-scheme: dark) {
+  body {
+    /* https://design.firefox.com/photon/visuals/color.html#dark-theme */
+    color: white;
+    background-color: #38383d;
+  }
+  #statusimg {
+    background-image: url("assets/status-off-dark.png");
+  }
+  #statusimg.on {
+    background-image: url("assets/status-on-dark.png");
+  }
+  #statusimg.on.running {
+    background-image: url("assets/status-running.png");
+  }
+  input:checked + .slider {
+    background-color: #cc80ff;
+  }
+  input:focus + .slider {
+    box-shadow: 0 0 1px #cc80ff;
+  }
+}