There is also the dark mode in Signal Private Messenger. However, this is hidden in the setting option. Just follow these steps to activate dark mode in Signal Private Messenger.
Step 1. Open Signal Private Messenger on your phone (Android).
Step 2. Tap your profile icon. Remember profile icon is located in the top left corner of the screen.
Step 3. Now you will see Settings page, tap on ‘Appearance.’
Step 4. Now Under Appearance, tap on the ‘Theme’ option.
Step 5. Here you will see two options: Light and Dark. To enable dark mode, just select the option ‘Dark’. Its done.