How can I VS2017 show the setting gear that can change the background of UI in XMAL designer?

Question

I am designing the Xmal UI with VS2017, There was time that I can see a gear above the XMAL Degisner, But most of time, I can't see it. In this place I can change the UI background between black and white. it's position is where above the designer, like in the screen shot below. Help me, please.