Posting in the Magento forums has been disabled pending the implementation of a new and improved forum solution which should better serve the community.
For new questions please post at magento.stackexchange.com, the community-run support site for the Magento community. We will be providing updates on the new forum solution soon. For questions or concerns please email email@example.com.
Do you know the name of the theme folder? You will need to find it and then enter it. I attached a screenshot to explain what Charles indicated. Insert the name of the theme folder where you see my_theme in the screenshot.
I think that you downloaded only Magento in Polish and not “lento” theme.... That theme (called “lento") is not yet released and the file you downloaded is a simple standard Magento translated in polish language.