Read the license.txt file in the themes folder!
When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.
Are there any other warranties I should be aware of?
Images and assets you download from websites or pages the web and use in your custom theme, might be not be able to be included with your theme, always get images from a proper source and check the license.
Even if you pay for Royalty Free licensed button images from somewhere like istock, these you cannot include with your template files even in a free theme, without purchasing an extended license.
Then there are some you can include in your theme for free or paid download.