How can I make a gui curve without styles?

I need to make a gui like this:

https://gyazo.com/9b92492571ae9f208a03b4d2b78da0ee <-- Click

I don't really care about the buttons.

But look at the blue background, it's curving? how do I do that?

This isn't really a scripting question... but I'm happy to help anyways!

The rounded square design on the blue background is most likely made with an ImageLabel. You can see an example of one by clicking here. I know the decal just looks like plain white but it's not. Use it as an ImageLabel over a frame for a curve design.

Where can I see more?