Hi - I am not sure if I understand "html boxes" do you mean divs?

If so to center a div, use margin in your CSS, for example if I wanted to create a box and center it on the screen my html would look like:



and my CSS would like:



if you are just trying to center the contents of a specific tag for example a p, do something like:



When you click the center button while editing content in Concrete, Tinymce makes code similar to the above.

so i would have too use css codes too centered it yes...

If it is a div...yes
You can do this through your themes's main.css file or the "Design" function when you click on a block.

Hope this helps.
Chris