Image-Line introduced theming to FL in 2022, but they used to allow you to do something way weirder...
Almost all of FL Studio's external organ (skin) was exposed to you as a collection of .BMPs, and you could play with them in whichever way your sick mind envisioned.
About
Internet Money was a really big producer collective on YouTube when I started making music in 2017. They were all on FL 11 because 12 sucked, and they all had this weird little skin they all used that was just FL 11 but flat and slightly darker.
I dunno what made me remember about the possibility of making one this year, but I became a little obsessed...
Essentially, how they work is: you take the default skin image files Image-Line hands you in the files that contain the (almost) whole UI; then you just draw over them. That's it.
It being that simple opens up a lot of fun and weird doors, since anyone with MS Paint could crack 'em open and have their way with the UI however they see fit without abiding by CSS syntax or anything like that.
I think I randomly decided to look up skins to see if I could download any, but couldn't find anything except screenshots. Then I saw a Gorillaz theme, and got put in a fucked up behavioral loop.
Turns out, you kinda need to know where to look for these, because the best googling gets you is a listicle page with some pics and links to dead pages.
One led me to a subforum that is long gone, which is fun in and of itself. But I also discovered a weird quirk some had; that being — you had to pay for them sometimes. I've never seen that be done in any other 'theming community', so that felt kinda crazy.
Maybe it was more normal in 2008 and if you wanted to make the background color of Yahoo! Mail pink, you were also expected to shill out 4 bucks, but that just kinda seems crazy today hdfhkjfadjhdjkh.
After some more looking, I managed to dig a bunch out from DeviantArt and a lone Dropbox link on a YouTube video.
Remember how I said most of FL's UI was exposed as BMP's? That bites a lot of these in the ass since there's a lot of stuff you can't theme.
Because of this a lot of the skins are fairly limited, only doing the header-bar-thing and some other crap selectively. That being said, you can still get a lot out of a skin, if there are wallpapers bundled.
Noelle Holiday Theme
I felt like making one myself, so I did!
I pray to god anyone, who ever made one of these had a script of some sorts to help them not go insane because I think I'd run into traffic if I didn't.
The visual philosophy I used for this crap mirrors the Gorillaz theme pretty closely — stickerbomb some characters you like all over it and have it not look like original FL too much.
For the shit I decided to keep UI-looking, I ripped off macOS 9 and CDE pretty heavily because it's really easy to draw and I feel like Noelle would be into retro PCs somewhat diegetically.
Links
I had way more to yap about in the video, so you should give that a look if you find this interesting!
Download the crap I stole/made for this.