Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Dark GUI Skin – PSD Free.
Dark GUI Skin – PSD
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Dark GUI Skin is dark skin packed with 6 different color scheme and with source PSD file.
With source PSD file you can easily change skin color to suit your needs. You can watch tutorial here.
At least Photoshop CS3 is required for editing PSD source file.