Dear Katja,
If you mean the icon that you click on to set the current topic when in editing mode, then it will be an image. So, right click to get its properties to find out its name and path in your current theme, then edit with your chosen graphics application.
And as far as I am aware it cannot be used for topic 0 - probably as it defeats the concept of topic 0 being for announcments and course information - therefore would never be logically current.
Regards,
Gareth