It already looks centred in the middle of the web page! It has equal margins either side.
In a lower resolution, say about 1024 x 768, it would look much better and easier to read.
In fact if you are not using a fluid theme layout then it's better to stick with 1024 x 768 resolution.
If you don't know how to do this there's an article
HERE you can read which explains it quite well.
However, if you want to make your theme FLUID, by this I mean it will expand and contract to fit any screen resolution, then you need to change the #layout-table width to 100%
but fix the side-columns to stay with 210px, and change middle-column to 100% this will allow the middle section of your page to adapt to the various page views when it will take up all the page in some cases when the sideblocks are not present.
Hope this is what you are asking to do.
If you're not sure how to do this say so, and we can help you further.
Cheers
Mary