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There are two areas in preference that deal with themes. One is "selectable themes" the other is "default theme"
selectable themes defines which themes are available in the theme block. To utilize the theme block you must place the block and make it visible, via that blocks administration menu.
Default theme declares the default theme for guest of your domain. Unfortuneatly certain XOOPS versions give users the oppurtunity to decide what theme is default for them. This may be your issue. To resolve it, go into preferences and unselect all of the unneeded themes from the selectable themes menu, leaving only the theme you want to make default.