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I solved my problem. The thing is, I read the long installation instructions, and also looked for any other documentation or forum help about how to change your theme in xoops but there's nothing about it anywhere I can see...
So I go to my administration panel, look for where to change Themes, and the closest I can find is Templates. So I click on that and see a list of themes and a choice to "Select theme". And pick the theme I want.
But this is not where you pick themes. Coming from Wordpress or Joomla this is confusing imo.
The place to change themes in Preferences, General Settings. It is like this in phpbb as well, so it does make some sense.
The main problem is that when you go to
https://xoops.org/modules/extgallery/ it doesn't say word one about how to actually use or set the themes. Also, doing a search on xoops.org doesn't come up with that information either.