This is a framework for Kerbal Space Program that allows you to replace HUD/UI textures at runtime.
GitHub | CKAN
These are not included in the download and must be installed separately (or you can just use CKAN).
Documentation on how to create texture packs, as well as how to recolor other UI elements, can be found in the repository wiki.
HUDReplacer is available under the GPLv3 license.