Sorry about the late response.
Since as far as i see the decadence in this game is not of the purely post-apocaliptic one, a good deal of the feeling can come from pretty simple things. Like in fallout, one of the things that screamed decadence for me in that game was the use of caps as money, not for the act in itself but for the feeling of a society that lost all contact with wherever they come from. The same thing with the Vaults, that began like temporary refuges and ended being microcosm all in themselves, with the shift in consciousness making them more twisted that the mad world outside.
But enough examples. There are some ways of showing decadence through graphical means i can take from the top of my head, without knowing more about the gameworld and it's background.
One is the use of well established symbols of the gameworld completely out of place. Like the use of structures that were most important to the culture now in decadence in a way that's logical given the setting, but almost an insult to the meaning of such symbol. The problem is that decadence is mostly a subjetive consideration, and as such is deeply rooted on the world that came before the decadence. A good way i think would be to see what paralels you can find in the real world to places, symbols, archetypes, et cetera and then how would it be decadent to use them. As long as the original meanings are well exposed and are able to get the player's sympathies it would spell decadence to see them used in such a way.
Another way is through imitation. If you have some notorious styles for structures of the old world, for an example, make some newer structures show a very similar style but simpler and rougher. Emptier, like they had the idea but not the soul. Or a noble could have his house filled with works of art and artifacts that are ordered without any rhyme nor reason in what they mean, put together just because they are rare and not cheap to come by. As far as the "fluff" for the meaning of those items is made clear long before the player encounters them it should be dandy.
I am not with the "dirtier" look. After all, the roman decadence began long before the empire came crashing down. I am more for the subtler way: A world somewhat shinny that has lost it's meaning and soul. A sage keeping thousands of ancient tomes and relics but unable to separate the gold from the lead, a noble obsesed with some old ideal or philosophy and trying to express it in every room of his manor - but getting it all so blatantly wrong it hurts. The peasants using some ancient symbols in banal ways, like having a post-nuclear world where the white house's obelisk is used for posting notices of interest to the town built around it. Some commoner dressing in a way that long before held great meaning just because that is what he could salvage and thought it was cool. And think how shocked most would be if they walked into a room where a collection of busts and pictures was kept, just to find the faces of their personal idols of human history just along those who they consider an insult to the species, side with side without any understanding of what they did and what separated the formers of the later.
Like a samurai who walked in a japanophiles convention, or a knight who did so in a medieval fair, and saw how those symbols and things that he hold dearest in his heart are now the toys of people unable of understanding what they meant and that does not even try to do so.
I hope this is useful to you in any way.