I highly recommend using Firebug or the Chrome inspection tools. Then you see the classes being applied to a table. Unlike what Arc suggests, I highly recommend not to use browser specific styles! Internet Explorer has been rightfully hated for doing this and I don't see much reason why webkit or mozilla based browsers should get away with it... non standard's compliant css is a pest.
Having said that, here's what you do...
If you want a table centered, you either use...
|centered|k
|cell|cell|
...with the following styles in your StyleSheet...
.viewer table.centered{
margin:0 auto;
}
...or...
{{centered{
|cell|cell|
}}}
...with these styles...
.viewer .centered table{
margin:0 auto;
}