Add quick theming tool.

src/style.css

@@ -641,6 +641,38 @@ dnd.playpen-tab-drag {
   border-color: @pp_border_strong;
 }
 
+/* The contrast slider, for the same reason as the swatch above it: Adwaita
+   paints a scale in the *desktop's* accent colour, which on a page whose whole
+   subject is the colour this window is painted from would be the one thing on
+   screen ignoring it. Left stock, dragging it towards a warm theme leaves a
+   blue bar sitting in the middle of the result. */
+.playpen-contrast trough {
+  background-color: @pp_surface_raised_active;
+}
+
+.playpen-contrast slider {
+  background-color: @pp_accent;
+  border: 1px solid @pp_accent_strong;
+  box-shadow: none;
+}
+
+.playpen-contrast:disabled slider {
+  background-color: @pp_border_strong;
+  border-color: @pp_border_strong;
+}
+
+/* The tick at the neutral position, and the two words at the ends. Faint on
+   purpose: they label the ends of a scale whose real readout is the window
+   behind the dialog. */
+.playpen-contrast marks indicator {
+  background-color: @pp_border_strong;
+}
+
+.playpen-contrast marks label {
+  font-size: 0.8em;
+  color: @pp_text_faint;
+}
+
 /* -------------------------------------------------------------------------
    The startup list