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For blank pages you need to switch on phpdebug in general preferences and paste the results here. See XOOPS for dummies and the "Trouble" section.
Check the themes are uploaded correctly to:
yoursite.com/themes
Sometimes when you unzip you get two folders i.e.
themename
themename(upload this one with the theme.html inside)
When you change themes to default in general preferences, you should also do this manualy by activating the "themes" block in system admin-blocks. (remember to also make your new theme selectable in general preferences, this makes it a selectable option in the themes block) XOOPS remembers the last theme you were using while you are logged in, whether you change the default theme in general preferences or not. Manually changing your default theme through the themes block changes your own personal default theme in the database.
Xoops also has a cache folder called templates_c, try clearing this out.