Open Snapchat
Tap your Bitmoji or profile icon in the top left
Tap the Settings gear in the top right
Scroll to App Appearance
Tap App Appearance
Select Always Dark
If App Appearance is not available, update Snapchat to the latest version
On iPhone, you can also use system-wide Dark Mode in iOS settings
On Android, if Dark Mode is not available in Snapchat, enable Dark Mode in your phone settings if supported