But all t3 items have same style w dark bg. And nova stands out a lot. Look at pidjan's screenshot. Ugly af
That's not a general rule. Just take a look at other item trees, specifically the items that look closer to finished.

I guess Toxic Bullets (a tier 3 item) must be such an eyesore with that dark background, despite being a candidate for the single best designed icon to have came out of the item update.

Or Escalating Resilience (another tier 3). My god, this don't even look like a weapon item, especially with all that green!
Here's a few example of items in the tier 3 vitality that does follow your design convention of a similar background color. Notice anything in particular?

(i will admit that this icon does radiate quite a bit of soul).


(and it's literal gameplay counterpart for good measure)
They're obviously incomplete. Far more so than Toxic Bullets or any other visually goated icon.
My main point is that the items which don't follow this background trend, tend to be far closer to completion than items that do. In my opinion, the developers should focus on making visually appealing icons. Making every background the same flat color with a bit of photoshop noise because some internet guy hallucinated a pattern when there is plenty of evidence for the contrary is not a recipe for a well-designed icon.
As more and more items receive updates, you will slowly see the flat colors go away, in favor of visually appealing designs. We will have less debuff removers, and more toxic bullets as time marches on.