Considering it is your first theme made for PokéCommunity, I have to give you props. I'll start by saying that your head banner is absolute eye candy. I love everything about it, and the text is remarkable. Also, the grey color scheme definitely fits the whole theme. By the way, I do like the button design choice.
Though, when there comes something good, expect the bad to follow. I have to admit that the background could have been something more, the small repeating style isn't very fitting for it as it seems kind of bland. Also, like Andy mentioned, the fixed width isn't really appealing enough for the theme. Other than this, I can agree with the rest that the only problem about the banner is that is just needs to blend in more with the theme.
Regardless, I'm liking it for a first and maybe we'll see some more in the future. I'm keeping my fingers crossed on that one, kay? ...