@samgaw, for your reference, here's why we adopted an MIT license for releases.
While I want to answer your legal question, I'm not a lawyer and don't feel qualified to provide anything official. How about this: I'm not aware of anything legal right now--speaking unofficially, as someone who's not a lawyer, who could be 100% wrong--restricting the use of different logos if you're modifying and compiling from source code.
@tom, definitely appreciate your feedback. I think theming--where elements are customized without obscuring software identity or usability--is desirable.
Your point on only having 3 votes on the feature idea forum is also valid. That limit was set in the early days of the project, when there weren't many features on which to vote. We're planning to increase the total votes per user to 10, and maintain the maximum of three votes from one user per feature.
Regarding hamburger menu, that's a tricky example. On the one hand, it's not about security at all and it's more about customization. On the other hand, there are certain elements in the UI, like the three dot menu in the desktop version, that are extensively documented and changing the icon would make the product documentation extremely confusing, which breaks FOF principle.
@tom, @samgaw, you're spending considerable time and energy on this, which means you care about Mattermost, and that is a huge compliment to everyone who works on the project.
I think on one end, there's the desire to personalize and customize Mattermost to make it look and feel welcoming to its end users. I think on the other end there's concern (at least in my mind) about security and the need for customers and IT managers to know what's running in their environments, and about usability and troubleshooting when the wrong kind of customization breaks documentation and makes end user issues difficult to troubleshoot.