Slack just got a new design with a much more purple sidebar in an update. Here’s how to change Slack’s sidebar colors back to the classic theme you’re familiar with, or pick any other color scheme.
Open Slack’s preferences by clicking the name of your team at the top left corner of the Slack window and clicking “Preferences.”
Select the “Sidebar” option on Slack’s preferences screen.
Slack’s new default theme is Aubergine, and you were automatically switched to it if you were using Slack’s original theme. Select “Aubergine Classic” to go back to the classic colors.
You can also select any of Slack’s other built-in themes here. Any theme you choose will only be seen by you, and everyone else in the Slack workspace can choose their own preferred theme.
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Source : How to Change Slack’s Annoying Purple Sidebar Color
