Hello Kars,
Thank you for your question. Here is a link to a helpful article here in our support center that covers "Changing your template background colour in Joomla 3.0," it is part of our class on "Configuring the Protostar Template in Joomla 3.0."
If you have any further questions, feel free to post them below, or contact live Tech Support.
They are available 24/7 via:
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&
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Thank you,
-John-Paul