4
Many themes use tables. It shouldn't be a problem.
It appears the XOOPS default theme at this site uses tables.
PHPKAOX (I think it is still a theme that is provided with Xoops) uses tables.
Karate Redux Blue uses tables. Actually, I think the majority of themes are table layouts.
I would recommend learning not to use tables, not because of Xoops, but, because it is better xhtml.
But, tables are fine.