Stuck in "default" group! (Essentials plugin)

I played around with the config files and some other files (as told in tutorial videos), so I could change my name color and add a colored prefix to my name. But when I was done, and went into my server, I always appear as "[default]" when I write something. I tried adding myself to the "Owner" group, but nothing happens.

I find it much easier if people ask for certain information, and I then give it to them, rather than me just typing our everything I did. This is mostly because I am likely to include something unrelated to the issue, and it might be considered as the problem, which will make the solution come much slower.

If you know your stuff with the Essentials family, just ask me the essential details (no pun intended), and check whether I messed up any specific part of the files. Rather than me just giving you a load of useless info to look through.