Recently, I had a lot of free time, so I decided to make some "graphic" modification of our CC. ^^

Screen #1





Screen #2




I would like to turn this plan into effect, but my programming skills are still at to low level .

Is this a mockup or the real thing? Either way, looks good.
One quibble I have with it though, the gradient shading on the menu looks off. I'd say either tone it down or have it come in from all corners. Or both.

I'm going to put my neck out here and say that's just a mockup. Remember all sprites have to be in palette, even in menus, so the gradients wouldn't be that pretty in game.

And also there's no way to make menu like that. When it comes to changing in-game menus, you are restricted to little to no possibilities.
Most you can do is to change a colour scheme and fonts afaik. Even then you are restricted to a certain resolution of fonts.

EDIT: However, you can change background and game logo to whatever you want it to be, just consider what Roast Veg told you - everything should be in palette. And same there, prepare to face some difficulties with size of your BG and resolutions.