How do I change my theme in Skype?

1. Select your profile picture.
2. Select  Settings.
3. Select  Appearance.
4. Under COLOR you can customize messages by chosing a gradient color or choose the classic color in Skype.
5. Under MODES, choose from Light, Dark, or Use system setting, and a check mark  will show next to your theme choice.
   
   Note: Choosing the Use system setting option will make Skype reflect the Dark or Light settings of your operating system. This is available for iOS 13 and above, Android 10 or above, MacOS, and Windows 10 and above.

Android 4.0.4 - 5.1

1. In Chats, tap the  Menu button.
2. Tap  Settings. 
3. Scroll down to toggle Use dark theme on or off.