I'm not sure the logo of steemit is actually fair game for something like that. While the blue logo of the currency that you can see below is open-source, I think steemit own this one and might not be very happy it is being used like that. Technically speaking, you need their permission for something like that. Practically, I bet they will not go after you, bit I still decide to let you know. You can read what they wrote about their new logo here.
Oh, Thank youx for that information. I will redesign t-shirt.
I really don't know if this is a problem, but just wanted to let you know what I've read from them :)
Thanks friend, i appreciate that!