Original report: https://github.com/mattermost/mattermost-server/issues/16268
It seems that slack has changed their format of exporting colour schemes.
I was able to reproduce on prev.test.mattermost.com as well as community servers.
Steps to reproduce
Try to paste this Slack colour scheme: #1A1D21,#121016,#3A2D3B,#FBFAF7,#27242C,#FBFAF7,#2EB67D,#BD3855,#4A154B,#FBFAF7
Receive an error "Invalid format, please try copying and pasting in again"
Verify importing themes form Slack - theme applies and displays as expected.
Note UX will need to determine the key value pair mapping. Perhaps this should be considered after we figure out what our approach to the new theming should be (if we expect major changes)
Grooming notes: I've attached a screenshot of the new theme options that Slack has added, so we need to support them as well in whatever way we've done in the past. If possible, we should ensure the theme still looks good if we need to infer other theme colours from the ones given, but we can further discuss that if it's not obvious.
The corresponding theme is