How to customize your Slack theme

From your desktop, open Slack and click the dropdown menu in the top right

Select 'Preferences' from the drop down

Within 'Preferences' select 'Themes'

Scroll down to test and select from Slack's predesigned themes

Or, if you want to customize further, select 'Create a custom theme'

Enter a six-digit HTML color value to each box or choose a swatch from the color picker