The "Powered by" text does not really depend on the Donut theme. It comes from the core and the theme just adds its own attribution.
You can modify the text of the attribution in these files (most likely you just care about the last one):
$lines='<description>Powered by Question2Answer</description>';
$attribution = '<!-- Powered by Question2Answer - http://www.question2answer.org/ -->';
'Powered by <a href="http://www.question2answer.org/">Question2Answer</a>',
Anyway, there is no need to remove it as you could just change it into something like "Thanks to". That won't match in the bot search results and you would still help Q2A.