Replace a Header?---FIXED

[reidluver]

Hello--I am incredibly new at this whole CSS thing and I'm doing my best to learn, so I apologize if anything I ask seems elementary.

I'm using a Mixit layout (Bookworm Blue) with a Plus Account and all I really want to do is change the header. I like absolutely everything else about the layout but I just want to change my header. I looked here and it gave me a CSS code which I then used appropriately. The image showed up...but it didn't replace the header I didn't want anymore. If this won't work simply because I have a Plus Account then I apologize for taking your time.

What can I do differently to just replace the header?

Here's the CSS code that I used:

#container{
background: #000000 url(http://i295.photobucket.com/albums/mm155/Reidluver/Firefliesheader1.jpg) no-repeat top center;
padding-top:300px;
}

Thank you very much for your time.

EDIT: Thank you to josiefier for helping me with the problem!