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Most themes work on all XOOPS versions, the way they are designed and that is why you won't find much info about it. (Some may need still some tuning and tweaking, but that is one of the pleasures of creating sites.)
However, if you plan to use some recent features, it is possible that the theme does not support this.
Example is the placement of the center right top or center left top blocks. The changes for incorporating these extensions are mosttimes easy, on condition that the graphic placement in the layout is possible. (search for theme column)
Some themes might also contain templates for formatting the content and/or blocks from modules. If the modules support in the meanwhile other data or features to show, it is possible that these templates need upgrading, which can mosttimes be done easy by comparing the content with the current template supplied by the recent module. (search for theme overloading)
However, if you plan to use some recent features, it is possible that the theme does not support this.
Example is the placement of the center right top or center left top blocks. The changes for incorporating these extensions are mosttimes easy, on condition that the graphic placement in the layout is possible. (search for theme column)
Some themes might also contain templates for formatting the content and/or blocks from modules. If the modules support in the meanwhile other data or features to show, it is possible that these templates need upgrading, which can mosttimes be done easy by comparing the content with the current template supplied by the recent module. (search for theme overloading)
now yes. 
