Ok, I've found a location of this new mainmenu graphic. It's in sharedassets12.assets file and it's called "mainMenu_bg_PX2.png". Sadly there is an animation as well on main menu (snowflakes, steam etc) which I don't know how to get rid of.
That's all I can do for now.
EDIT:
Ok, I did it. KOTOR style
Instructions:
1. Open resources.assets with Unity Assets Bundle Extractor.
2. Find those textures:
mainMenu_bg_PX2 - image background from White March
fx_blobs01 - snow flakes
fx_smoke_plume_burst_02 - smoke which looks weird without snowy background (you need to edit some texture settings with it as well, mine looks like this:)
3. replace those textures with some transparent one.
4. Save files, and replace original ones.
You can also replace background image to PoE1 Background if you like.
Enjoy