Ghost is free software released under the MIT License, which pretty much means you can do anything you want with it. The MIT License is one of the most free and open licenses in the world, and does not restrict how you use the software which it's applied to. We believe open source software should be free. As in free.
You are allowed to use Ghost however you like, so long as the copyright notices remain in tact. With the MIT license you have the same freedom to decide how you want to engage with Open Source software. You decide whether your theme or plugin is MIT, GPL, or any license you like.
It's up to you to decide what's best for your project. We believe giving people a choice is the most powerful statement a license can make.
The full license is embedded below, and ships with every single copy of Ghost.
Copyright © 2013 - 2017 The Ghost Foundation
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.