https://www.wowinterface.com/downloads/info25114-modui.html
Click the M above the backpack button (bottom-right of the main actionbar) to open theme and display options. modui is designed to be modular and the different elements of the UI can be turned on and off.
Q: I'm getting an error that says something about MODUI_VAR.
A: try reloading the UI (type: /rl) and then deleting your cache before making an issue, particularly if you've recently used an older version of modui.
Q: The UI won't load in my game!
A: Make sure you're putting the directory in WoW/Classic/Interface/AddOns/, then rename the folder from modui_classic-master to modui_classic.
Q: Why aren't my target and nameplate casting timers correct?
A: Blizzard recently disabled accurate timer checking in their classic API in the run up to release, so now its no longer possible to check which spell rank is being used - so modui has to assume its the highest rank with the longest spell cast time for now.
Q: I don't want a black UI, how do I change it back to normal?
A: Open the theme option by hitting the M above the bag button or typing /modui, then choose UI Colour. Hit Reset To Default or move the colour wheel picker to white and hit Okay.