I would advise you not to make any changes. The screen looks great according to the screenshot you've sent. On such a big screen, it's very good like that. Sometimes we want to have our frontend so good that we start doing very unnecessary things, and I know a lot about that, because I've been wrong many times.
At most, what I would do would be to center it on the screen, and if you want to see everything black behind, you just have to use a jpg or png of 1 pixel white and paint with it the whole screen in black or semitransparent with the alpha level you want:
local flw = fe.layout.width;
local flh = fe.layout.height;
local search = fe.add_image("objs/pixel_white.png", 0, 0, flw, flh);
search.set_rgb(0,0,0);
search.alpha = 255;
But I insist that I would do nothing, I like it as I see it.