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Well, part of it is now being included in XOOPS 2.5 Alpha 1, in the file manager.
The first one, you can easily select a theme by going to
http://www.xoops.site/index.php?xoops_theme_select=nameoftheme.
So if you go to
https://xoops.org/index.php?xoops_theme_select=default you'll see the default theme.
You can make a content page and add pictures of all of your themes on it and get such a link.
But you can also use the theme block and use the function to let it show a thumbnail of the theme. That's most likely what you mean.
Duplicating themes can be done, but not with every theme though. Some themes use this: <{xoappurl>/themes/nameoftheme/ (or something) as link, and you can't rename them or you'll break all of the images and stylesheets in them. Most themes can be used with changing the name though, but not all.
And changing a theme is more than just editing themes.html and css - you often have (with Morphogenesis themes) a lot of templates in the xotpl folder. That makes it very hard to edit them if you're just starting. XOOPS theming is quite easy, but it's not so easy that everybody can do it - you'll need basic knowledge of css and html - and not everybody has that. Even I have problems often when making a theme (most of the time for XOOPS, but I've ran into problems too when making themes for Pluck. Concrete5 is easy.)
A themes module isn't coming soon I fear - but I have heard from something that Aphex did with a theme. I thought you could edit it with debaser. You can read that here:
https://xoops.org/modules/news/article.php?storyid=5148Maybe that's what you're looking for?