GTK 3.20 :: fine-tune indication almost invisible

[SammysHP]

If you long press on a slider handle the slight background color change is hardly noticeable. Please add some better indication for fine-tune mode.

[megax]

@SammysHP You do not do a PR? :)

[SammysHP]

I might do it. Just porting my personal changes¹ to the current master and see if I get a good idea.

¹ Actually it's quite interesting that there's not much difference between my version and upstream. A few months ago I changed a lot, like button sizes, various margins and paddings, colors and assets. Now it's just the headerbar/menu background color, the headerbar margin and assets (which I need to fix after all the moving and exporting of assets).