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Still havn't found theme.html?
First you need to work out what theme you're using
Go to admin menu --> preferences --> System --> General settings, then about half way down the page, it has deault theme.
Remember the name of the theme sated here.
Now go (in a FTP program) to yoursite.com/themes/<themename/>/theme.html
That is the file you need to edit.
Edit it and upload it and your site should change!