The color seems too grey. You should make it a little darker. The bar on the side serves no purpose, unless it is a styled notification theme. But this seems like it’s a main menu window.
The partly visible UICorners don’t look very good.
The OK button’s drop shadow lacks in contrast and is too subtle.
I can see which kind of style you’re coming from, but I don’t particularly think of this as modern.
Here’s a few things. Grey is very serious, most players here are kids, consider if serious is the mood you are going for.
The inner box is cut on the right, or looks cut to me. And I’d maybe use less white lines, or lines in general, they don’t really add anything, more of a distraction.
Look at modern mobile game UI, its optimized by years of cash grabbing