This is not legal advice
When you upload a theme to OpenUserCSS, you have a choice of applying
a license to it in order to permit more generous use of your theme.
For your convenience, this site provides a list of licenses written
by people that are not me, nor related to OpenUserCSS.
This list contains licenses that are popular in the Open Source
community and are approved by the Open Source Initiative.
OSI is also not related to either me or OpenUserCSS.
Of course, you always have the right to apply no license to your
theme. Keep in mind that by uploading your code to this site,
you're providing an implied license for the public to download and
use your theme for non-commercial purposes.
You may also choose to apply a license not available in the list of licenses. To do this, choose the "Other" option in the dropdown. In this case, it falls on you to display your license text in a prominent position in your theme description.
I'm not a lawyer, nor does OpenUserCSS provide legal advice to anyone. If you're concerned about copyright, please consult with your/an attorney.