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Make sure the unpacked zip file doesn't contain several nested folders. Only the single folder with 'theme.html' in it should go into the Xoops 'themes' folder.
In Xoops administration, choose 'system/preferences/general settings' and you should see the theme name appear in the 'selectable themes' field. You need to control-click on the theme(s) to highlight them, then set a default theme, or activate the 'themes' block to allow users to choose a theme.
Standard troubleshooting procedures are to delete the contents of the 'smarty_cache', 'smarty_compile' and 'xoops_cache' folders in xoops_data/caches (the files will be automatically rebuilt). Then open 'system/modules' and 'update' the system module by pressing the small 'box' icon with a green light.
The only time you might get a non-styled page is if you access your Xoops site from another PC on a local network. The theme won't work if any of the paths in mainfile.php are set to 'localhost' rather than the server PC's actual DHCP address (e.g. 192.168.*.*).
HTH
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