I had a similar problem -- had a clean install and then the first thing I tried was changing a theme and this resulted in a completely blank page. In my case, the entire site was completely blank -- I couldn't even log in -- so I cleared my cache, reloaded, and closed the browser entirely -- nada.

Here's what I did to get it back for any other completely perplexed newb like me.
1. went into mysql, there's a xoops_config table, browsed it and edited the var, theme_set -- I just entered "default" since I knew that worked.
2. this allowed me to see the login page, but then the admin page was blank.
3. I then killed my cache, and manually entered myxoopsdomain.com/admin.php and was able to see it. Why? I dunno -- maybe the cache?
4. then I again changed the theme to something else and it thankfully worked.
5. sheesh