Azure DevOps has a feature similar to Azure which you can use to change the background theme to either dark or light them. By default Azure DevOps gets displayed in light colored them but you can change into Dark theme by following the steps below
Step 1: Login into Azure DevOps and click on the user icon at the top right
Step 2: Then click on “User Settings” as shown below
Step 3: Then click on “Theme” as shown below
Step 4: Then clicking on the dark theme from the options as shown below. Once you click on Dark the entire background color and theme would be changed to Black in Azure DevOps.
To revert it back to the default light theme, Follow the same steps again and then choose the Light theme from options.