Slack just got a new design with a much lilac sidebar in an update. To change the colors of the Slack sidebar back to a classic theme or to select a different color scheme, follow these steps.
Open Slack's preferences by clicking on your team's name in the upper-left corner of the Slack window and clicking "Preferences."
Select the "Sidebar" option in Slack's Preferences screen.
Slack's new standard theme is Eggplant Slack's original theme, choose "Eggplant Classic" to return to the classic colors.
You can also use one of the other built-in colors here Select themes from Slack Any topic you select will only be seen by you, and everyone else in the Slack workspace will be able to pick their own favorite theme.
If you do not like one of these topics, you can You select a user – created theme – or create your own! Just click on the link "Customize your design and share it with others" at the bottom of the list of topics. 
Here you can select custom colors for Slacks theme.
You can find user-created topics on sites like slackthemes.net. Simply copy the values from the design website into the Custom Design box on your Slack settings screen.
For example, here's a pretty tron design if you want a dark black sidebar with blue markers. Simply copy the theme code from the website and paste it directly into the box in Slack's preferences. Your changes take effect immediately.
Want to share a cool theme you found or shared with others in Slack? That's easy too. Just paste the code into a Slack message and submit it. Other people in Slack see the Change Sidebar button, which they can click to quickly switch to their shared topic.
The theme you choose will follow your slack account across devices Your selected Slack design on all your PCs, Macs, Chromebooks, iPads, and even your phone.