You made every HTML and CSS mistake one could make, so I really suggest you look some things up and learn a bit more. Your custom theme is miles away from being valid or from looking the same in all major browsers. I edited out some of the most serious mistakes, like using header tags (<h1>, <h2>, ...) for actual size representation. The header tag is a logical atribute - it mustn't be abused for representing sections of your output in a particular font-size. Another very silly thing is relying on the presense of a particular font and throwing all fallback mechanisms overboard - your site looks really stange on other machines but yours, as "Trebuchet MS" can't actually be called a common font. Using the background image of the actor your site is about and tiling that tiny image all over the screen looks weird as well and gives your page a very "home-made" look (home-made as in "unprofessional").
I could go on about the design mistakes you made for hours, anyway that would be too much off-topic. Your question was "why does my navigation not look centered"? The answer is: the navigation resided inside a table that was set to use 100% of the screen width, with the table cell at the most right position set to fill up all remaining space. With that setup (a table that already is 100% wide), you can put any wrapper around that construct, the result will remain. I edited out the 100% width and got rid of the empty table cell at the very right - now you should get the intended look. However: before fine-tuning your site by looking into the alignement of your menu, you should make it work in visitor's browsers first, which means: fix your HTML and CSS, it's a mess. Use the modified theme I attched (and keep the theme name that I have given your theme to make sure you don't overwrite your custom theme when updating in the future) as a base. Then try to validate your output first - you'll learn a lot if you do so.