Customize the Appearance in Microsoft Teams Free

Select your profile picture. 

Select Settings .

Select Appearance ​​​​​​​. 

Select Light,Dark, or System Default.

Select Restart. When Microsoft Teams Free restarts, your theme will be changed.

Open Microsoft Teams Free.

Select Settings and more near your profile picture.

Select ​​​​​​​ Settings .

Select Appearance .

Under Theme ​​​​​​​, from the drop-down you can select:

1.  Follow operating system theme, Light, Dark, or High Contrast.

2. You can also turn Always use dark theme for meetings and calls on or off.

Your theme will change to your selection.