css: Put small L/R margins on speed scale

GTK does weird glitches when scale slider inside popover is dragged to the very end. Avoid that by putting small margins on both sides. It also looks better when its shorter a little bit.

css/styles.css

@@ -277,6 +277,10 @@ scale trough slider {
 .elapsedpopover contents {
   padding-bottom: 0px;
 }
+.speedscale {
+  margin-left: 4px;
+  margin-right: 4px;
+}
 .speedscale trough highlight {
   min-height: 4px;
 }