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Use MySQL front, phpMyAdmin, whatever to open your mySQL dB. Try going into the xoops_config Table to the _MD_AM_DTHEME field and replacing the value with:
default
that is just the word, no '"{( no anything just the word default. Then try hitting your index page.
or see if you can acess:
yourURL/user.php
login then go to:
yourURL/modules/system/admin.php
and change the theme..
GL