Interesting. I just downloaded and had a look at that. It is a single DDS edit, Suggesting that mods that only edit textures will not break achievements.I know that the Mod that changes the colour of neutral armies to grey is achievemnt compatible. I got an achievement with it on.
Best way to check would be with the mod on and trying to get an easy achievemnt. If it doesnt work try the same thing without the mod.
However, for me to make the dark UI I need to edit gui files, so we'll have to see what that does when I test it later this evening.