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Templates control the layout of individual blocks. Most existing XOOPS themes are based on the XOOPS "default" template.
You can't edit the default template, you must clone it first, and edit the clone. To clone the default template
Go to System Admin-->Templates, and click "Clone" next to the default template, you may call this clone anything you wish. XOOPS will now install the clone. After you have done this, go to System Admin-->Preferences-->General Settings and make the clone your default Template (not theme)
Then go back to System Admin-->Templates. a tick should be to the right of the default template. You may need to click on "Generate" if any of the module files have a "red" generate link.
Your Blocks layout is controlled from Templates, go to System-->Admin--Blocks and look for the block you wish to alter. Then click "Edit" to the right of the block. In the next dialogue box you will see "Edit Template". Click this, and you will see the code for your block. You will also see the "classes" that are being called which relate to the style.css sheet in your themes folder.
Edit:
If you specify the block you wish to center I might be able to assist with this