UI modifications are not software. They are "mods". Everquest is software, and if you can "plug in" some "mods" into your EQ folder, and EQ itself allows/runs them, then it's not a problem because no other software except EQ is running.
I wouldn't even go so far as to call interfaces modifications, and yes its not software. Its just textures and XML files that tell EQ how to display the existing elements within the UI. You are changing the look of EQ, not the feel. The fine line is does it make the game do something that it does not do on its own? If so then its not allowed per this FAQ: https://forums.daybreakgames.com/eq/index.php?threads/what-constitutes-as-cheating.281242/ Dropping files (like interfaces and maps) in a supported EQ format within a specific directory is allowed because that type of customization is built into the game.
