For some reason, the Slack app on Android and iOS is a little less customizable than Slack on other platforms, so you can't adjust the sidebar theme to different colors directly inside the app. But that doesn't mean you can't customize the look of your mobile app at all. It's just a little less convenient.
There are few apps or services better than Slack for keeping a team both communicative and productive. But it also makes it difficult to escape your job — after work hours and weekends should be yours, not Slack's. If you want to stop the barrage of notifications and messages on your time off, you should make use of Slack's "Do Not Disturb" and "Away" settings.