Increase transparency of Loop rectangle so tempo lines are visible

Should fix bug#6163

gtk2_ardour/dark.colors

@@ -487,7 +487,7 @@
     <Modifier name="ghost track base" modifier="= alpha:0.640782"/>
     <Modifier name="ghost track midi fill" modifier="= alpha:0.3"/>
     <Modifier name="inactive crossfade" modifier="= alpha:0.4666"/>
-    <Modifier name="loop rectangle" modifier="= alpha:0.9764"/>
+    <Modifier name="loop rectangle" modifier="= alpha:0.5"/>
     <Modifier name="marker bar" modifier="= alpha:0.5"/>
     <Modifier name="measure line beat" modifier="= alpha:1.0"/>
     <Modifier name="midi frame base" modifier="= alpha:0.4"/>