Usually, Android devices may not support Dark Mode on Snapchat, but you can try this alternative solution.
1. Go to your phone settings and Choose "Display".
2. Enable "Dark Mode" and go back to the settings.
3. Then Scroll down and click on "About Phone".
4. Choose "Software Information".
5. Find "Build Number" and repeatedly press it six times. (You will get an "almost there" notification about developer mode, then input your password/code to enable mode)
6. Now go back to "Settings" and click on the "Developer Options".
7. Then, Scroll down and select "Force Dark Mode.