Well, in the background itself, nothing really changed - it is more a question of focus, and in the past couple years GW focused almost exclusively on Space Marines (gothic grimdark) rather than places like Necromunda (80s cyberpunk).
Various redesigns such as new "Tempestus Scions" certainly do not help either, though ...
But I don't think the Tau make the Imperium feel any more evil or backwards than the Eldar already did, especially as it's rather easy to give the Tau a dark spin as well. The "Greater Good" is perfect for justifying smaller atrocities and/or infringements, as the real world has proven again and again, and is proving again right now with stuff like the Patriot Act etc.