How do I stretch a frame or edit the roblox top bar so it's not transparent?

So I want to make the entire screen grey but the bar on the top is always showing the world.

https://gyazo.com/636fd7a33d7fa4a1b1c7499f083683a9

Anyone know how?

If you do can you also tell me how people add categories to the top bar?

This will make your top bar transparency = 1. Place in a LocalScript

game:GetService('Players').LocalPlayer:WaitForChild('PlayerGui'):SetTopbarTransparency(1.0)

Source: https://developer.roblox.com/api-reference/function/PlayerGui/SetTopbarTransparency