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If you mean for when your site is closed not actually down, the site closed template is in modules > system > templates > sytem_siteclosed.html
You can edit it, then to use it you can upload it to your theme with this directory structure:
modules > system > sytem_siteclosed.html
And it will render immediately. This template overriding would require XOOPS 2.0.14 or higher, although it is best to be up to date and be running on 2.0.16
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