NoiceGRUB is a collection of linear gradient based GRUB2 themes that can be customised. By default NoiceGRUB comes with 4 templates and 8 presets.
For generating a theme of your own, visit the GitHub repo

Presets in the image view:
Mountains from "Mountains" template
GradientGuy from "TheMan" template
Noice from "Noice" Template
VioBlue from "Kewl" Template

Preview of other presets can found at the Github repo
https://github.com/1337kid/NoiceGRUB/