Slate is a new Open Source Skin Template designed to make osu! skinning easy.
The skin is currently in beta and only supports Photoshop.
By default, Slate has a prebuilt skin on the Source Files, but it can be easily replaced, most of the important files on the project, such as the Song Select contain a Master Template that can be used to change the entire interface by editing a single file.
Slate is currently in a development state, some things may not work as expected, or at all!
However, if you're willing to work with these limitations, and also help towards the development of this project, you can go to the GitHub Repository to give it a try.
It is also very recommended to read the GitHub wiki, which is also linked below, it should help with any doubts, and if not, don't hesitate to send me a message!