Support setting sliders brighter #13

When dark theme is used, sliders (especially progress on fullscreen) tend to look a little too dark (as if they were disabled). To overcome this, add an option to force them to be 20% brighter.

css/styles.css

@@ -156,3 +156,6 @@ scale marks {
 .gpufriendlyfs {
   box-shadow: none;
 }
+.brightscale trough highlight {
+  filter: brightness(120%);
+}