DISCLAIMER: SMUI is still in "pre-1.0" and this description may not accurately reflect the current features/state of development)
Providing a vast library of UI components, SMUI is designed to assist developers in creating/iterating on simple to advanced UI aesthetics and functionality. Additionally SMUI supports custom XML layouts for designing interfaces in a modular and declarative fashion (documentation can be found on the GitHub wiki.
SMUI is available on Nexus Mods or as a release from this repository
SMUI is available as a Nuget package or can be referenced from the SMAPI mods folder when installed as a mod.
Keep in mind that when SMUI is referenced, the build process will create a copy of the SMUI assembly in the output directory/mods folder. This can be disabled