Firefox not loading chrome CSS file

Greetings. Today I discovered the world of theming Firefox through something called CSS. I went to the store page, and I wanted to add the Opera GX theme. Therefore, I first enabled the legacy toolkit config in the about:config page. Then, I went to the UserChrome.css file found on github for that theme and copied the raw data. Here is where things get interesting. I went to the about:profiles page and tried to open the root directory of the default profile, and it failed to open. I then tried searching for the directory in the file manager, it does not exist. No, it’s not hidden, it does not exist! The local directory, however, does exist, so I opened it and made the chrome folder and subsequently the userChrome.css file by using the text editor. I then restarted firefox, and…nothing happened. I cleared my browsing data and disabled all my of extensions, still nothing. I then had the idea of creating the root directory myself, so I created the necessary folders in the correct locations according to the about: profiles page, and then I moved the contents of the local directory to the root directory, and then restarted firefox. And…nothing happened. I was scratching my head at the point, so I tried searching the web to see if anyone else had a similar problem to mine, but I didn’t find anything relevant to my problem. Has anyone encountered a similar problem to mine? Is there anything that I’m missing? My OS is Ubuntu 23.04 and I’m using Firefox 115.0.2 (flatpak). I posted my problem on the Firefox CSS community, but the problem with it is that it's not active, so I hope the mods don't mind my post here. Any help would be appreciated.