Despite the title, the actual skin used for the footer came from the OFF SCRIPT skin (slightly modified to be compatible on lazer).
This is just a proof-of-concept and actually pretty simple to achieve. Basically, since lazer already have "SkinnableSprite" on the skin editor, I just need to find a way to make it layered above the footer (which is impossible on vanilla lazer). The proper way is to modify the depth of skinnable sprite to be placed above everything. But inspector says that the sprite is placed deep inside a container where it live alongside (and before) the actual footer panel. So the skinnable layer need to be taken outside the container and live alongside (and after) the footer layer. But I got no time to do that.
Simple workaround is just simply removing the bg of footer which will make it transparent, thus making sprite visible. This will make it still below osu!cookie logo and the button, so the fix is just positioning away the cookie. The button is intended to be above the sprite on this one.
And yeah the rest is just bunch of small changes to match the skin. Like button and panel size, colors, removing the skew (diagonal style), making rounded rectangle sharper on all panel, etc.
Much of this is not a proper way skin lazer. The whole reason osu!stable skin does not work in lazer is simply about UI scaling (and the fact that the whole UI style changed). On the video, you can see the skin "broke" because it's getting shrinked; and it's worse if you have ultrawide screen. So skinning is quite complicated unless they wholely implement the osu!stable menu screen which will... look out of place.
Here's a video without the center gap, it may slightly looks better when scaling this way.