To install one, just navigate to the u-he application support folder and replace the file 'ZebraletteBack2.png' with one of these. On macOS is is located here: Library > Application Support > u-he > Zebra2 > Images > Panes
I was bored and just not a fan of the orange, so I made an attempt to create some skins

My first go was to just change the color. I then had the idea to try to add a Batman/Dark Knight theme, because of Zebra2 being heavily used in the Dark Knight soundtrack.
The downside of these (like the default orange) is the resolution is low (713x384). So when resizing the plugin over 100%, it's not so nice.
I had shared these with others, and Selig pointed out that the background pattern was a bit distracting. So I gave it go at creating a skin from scratch. These are simple and have twice the resolution so they can accommodate the plugin being resized to 200%. There's a blue flat, blue gradient, dark, and some batman themes. I won't post images of all. And the flat ones should be easy for you to make your own adjustments.