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 to different colors directly in 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 comfortable.
Mobile app, desktop app, mobile browser version and desktop version of Slack are all interconnected. So, although some things can be customized for each version, if you do not receive messages, such things as the sidebar color scheme can be the same for all versions.
You can carefully see where I go with this. Because there are no settings in the Slack app for iPhone or Android phones to adjust the sidebar, you need to customize them elsewhere, and the customizations for the mobile apps and where you choose to adjust them.
I will show you how to do it from a mobile "desktop" site (the regular mobile site does not work), but the process will be the same if you do it in a real desktop browser, MacOS app, Windows app or Linux app.
Step 1: Log in to the Slack Website in a browser
Visit slack.com/signin in the mobile browser of your choice, then log in as you normally would to your workspace and to your account. If you have 2FA configuration, you must also use an authentication code for verification. Again, you can also log on to one of the desktop apps or the website of your computer – it doesn't matter – the process below will be the same.
Step 2: Request the Desktop Website
As you can see, I request the desktop site before signing in, and that's okay, it doesn't matter if you do it before or After you log in, just make sure you do it before you go any further. The desktop page request process varies depending on your browser and operating system, so the following shows some examples using popular browsers.
Step 3: Open the sidebar settings
Now tap your name and work area at the top left of the sidebar, giving some options, one of which is "Preferences." Select it and then select "Sidebar" settings on next page.
Quench comes with seven original themes as well as two available themes for users with protanopia and deuteranopia (both for red-green color blindness) or tritanopia (for blue-yellow color blindness). Eggplant, Eggplant Classic, Hoth, Monument, Choco Mint, Ochin and Work Hard.
You can choose any of these and as soon as you do, your sidebar is changed to use them – both on the desktop side you r and the Slack mobile app. There is an almost instantaneous update on all platforms.