I think I have never seen someone release a custom button set and if I see buttons in games, they are default.
But do they really have to be?
Since they look pretty generic, the default buttons work with any type of game, here you can see them “in action” in the menu:
But if you have a very specific game and setting, maybe you want to adjust them to better fit your vision?
Plus, the default buttons work well with the default and font, if you change those two, you migth want the buttons to match! Especially if you choose a more "retro" and pixelated style, unique buttons can do a lot!
But how do they look like as resource?
The buttons themselves are laid out in a 48x48 grid, with the return and the checkbox being double wide. There are also two rows, the upper one being the buttons you see displayed and the turquoise one being the one which replace the upper row icons when clicked on.
Within those limitations, anything is possible. You can search for premade parts to build the buttons you like or go in from scratch.
Tip: if you look e.g. at opengame art, you can get nice inspirations or bases for edits by looking for "gui":
Turning Polyhedron GUI Various GUI Elements Various GUI Elements - Part 2 Simple RPG GUI
I for example used my own GUI icons from my MV/MZ iconset as a reference, since I liked the color sheme.
Since you just need two basic shapes, those are really easy to create!
The symbols are very generic and easy to draw as well:
And then, with some simple shading, we already have some unique buttons:
Since those are our own unique buttons, we can pick any “clicked” effect that we like - it does not have to be “just” a color alteration. How about having the button move “into” the screen, as if it was pressed down? You can either stick with a transparent overlay as the default did, or you go for a solid image that completly covers the default as I did here.
Added to the game, they add their own new flavor to the menu:
You are free to do what you like here, just keep in mind that they are in fact customizable and those minutes invested can pay off a lot. You could even use elements of your windowskin to create them and have them blend into the other UI elements!