From the look of the screenshots ( http://www.mattermost.org/70-2/ ), I'd say there are some copyright concerns with the styles. This isn't a Slack alternative, it's a Slack clone. That's very problematic from a copyright stance. Why not create your own design style?
why? they didn't copy slack (i assume) - it just looks similar. Slack doesn't (and shouldn't be allowed) own a look and feel, as long as the implementation of it is cleanroom.
I have no idea if Slack could actually win a case, but in the US (where both companies appear to be headquartered) they do have some rights to look and feel as part of trademark law.
Interesting. There's potential for that as a basis for a civil suit, but i really doubt that the open source alternative could really be argued as trying to misrepresent or cause confusion as to the origin of the product. It'd be a hard case.
I think you misunderstand copyright. You can't copyright a style, only a specific implementation thereof. Unless you're accusing Mattermost of copying code directly from Slack (which you wouldn't be able to tell from a picture), there is no copyright violation.
From the look of the screenshots ( http://www.mattermost.org/70-2/ ), I'd say there are some copyright concerns with the styles. This isn't a Slack alternative, it's a Slack clone. That's very problematic from a copyright stance. Why not create your own design style?
Gogs had (has?) this same issue: https://github.com/gogits/gogs/issues/1069