Set theme preference for each team
Another idea from Slack, but one I miss more and more. In Slack, you can set a personal preference on what theme to use for each team you are signed into. I guess this would require more themes other than the light/dark/high contrast, but having this ability makes it a lot easier to determine what team's channel I am currently viewing without having to read anything.
+1 but it would be great to also have a company-wide colour palette +1 e.g. using #XXXXXX hex input or a simple colour picker/selector so that admins have the ability to pre-select the company's or department's theme