Material UI replaces FFXIV’s built-in theme (dark-theme) 4K variant.

This requires you to have the XIVLauncher and Penumbra installed.
The repo for Material UI needs to be added to Dalamud:

MaterialUI Repo: https://raw.githubusercontent.com/Sevii77/ffxiv_materialui_accent/master/repo.json

The Material UI config can be run with /materialui
Select your accent colors and activate any additional mods. 
Select ‘Apply’ and give it a few minutes to download. 

Find the mod Material UI in Penumbra, and enable it, selecting any of the customizations needed. 
It would be better to create a separate collection for UI mods and then applying it on your “Interface Collection” in your collections.