There are various background colors/themes you can change in Azure Portal. To change the Azure Portal to dark mode follow the below steps

Step 1: Go to Azure Portal

Step 2 : Click on Settings at the top

Step 3: Then Click on Appearence+Startup Views

Step 4: Then choose the dark mode.

Step 5: Click on Apply

As soon as you have clicked on Apply, you can see the Azure Portal in dark mode as shown below

But dark mode is really bad for your eyes..so try to switch to a white mode rather than a dark mode..