Player template colours

Hi, could anyone please tell me where I adjust the blue colour that can be seen in the attached image? 

I can't see this blue colour anywhere in my player template listings, i'm wondering if it's associated with the theme colours.

It doesn't appear when you click the tab normally, unless you click on the very very edge - it seems a bit glitchy and i've only just spotted it after months of using this player.

Thanks!