So, beyond being poly, I have over ten years experience living with roommates, and I wanted to offer some thoughts, based on my experience there.
First off, in every, single place I've lived since I was 20, I was either the primary leaseholder, or a co-leaseholder. So I'm coming at this from a place of being one of the people (or THE person) who was in charge of the home.
I have had roommates who mainly wanted to live with another person to have a crash space/place to store their stuff, but spent the majority of their time out. They had their bedroom, and that's all they had, and they weren't particularly invested in the apartment or me. Even if we had a great time when they were around, we were casual friends at most, and they didn't feel particularly invested in the home.
I have also had roommates that really wanted a *home*. A place that felt like theirs, wholly theirs, not just "this part is 'mine' and everything else is 'not mine'". For those people (especially the ones who up front signed a sub-lease saying they would live there for a year, and possibly would want to live their longer), I asked them "what do we need to do here, so this feels like your home too?". It didn't have to be equally 50/50, but it was important to me, as a person sharing my home (and my life) with another person, that they felt like they had a home too.
What we did in various cases - at the least, some of their pictures went up on the walls too. Some of their decorations came out. But at the most, with the person who ultimately was my partner for a number of years (Rachel), we repainted two rooms, got rid of a couple of old pieces of furniture, and picked some new out together, and some pieces she bought herself, that would be hers.
And the home felt like ours. Even if much of it was decorated by me originally, having her stuff in there too, that meant a lot to both of us.
For a bit less than a year, we sort of had a third roommate, who was a good friend of ours getting out of a bad breakup (there were only two bedrooms, and we were all close enough to sleep together, so we all kinda rotated through them, depending on who needed space, and who was sleeping at another partner's, or who had a partner sleeping over). In honor of our 3rd roommie breaking up with a really bad boyfriend, we also put a lot of her stuff on the walls, repainted a couple of trims, and added a few pieces of her furniture (I have a small storage unit, and it's easy for me to swap furniture in and out because of that). When she moved out, Rachel and I did a bit of redecorating/moving stuff about again.
So I'd suggest, based on that experience of living with other people, that if you're planning on being with them for a long time, why NOT participate in the decorating a bit? You use the kitchen, right? And the living room, the bathrooms, the backyard? Why WOULDN'T you participate in *some* of the decorating?
Have you brought that up to her at all? Are your styles compatible? What would feel good to you, or mean the most? Maybe having a couple of mugs that you love to use in the kitchen? Or if you spend a lot of time in the living room, maybe you and your meta could pick out the paint colors there together? Perhaps buying a blanket for the living room couch (or pulling one out that you already own and love) would help.
If you're living there full time, I really think you deserve to be a bit more involved in decorating the home, especially if you all plan to be there for years. IMO, as someone who's lived with a lot of people (and always been "in charge"), redecorating a little so it feels homey to someone else living there is very worth it, for the good feelings and love that it engenders. If I were so rigid about my decorating that I couldn't bear to have a few nick-nacks and a bit of furniture that wasn't mine in the home, then I shouldn't really live with other people.
I feel like a couple of questions like this have come up lately. If we've got a Secondary Bill of Rights that we talk about, in terms of basic human rights that secondary partners should always have, it would seem that we should also have some kind of Secondary Partner in the Home Bill of Rights, reminding us that if we can blend partners, and love lives, we can surely blend some furniture styles and paint colors.