The type of UI rendering that works is dependent on your OS client build:
To turn ‘Always Dark' or ‘Match System' on or off in iOS: Edit your profile picture from the profile page, click on Settings (gear icon) > My Account > App Appearance.
On Android: On the Bitmoji app click the settings gear, and then 'Always Dark'.
Keep in mind that Dark Mode used to be a hidden feature behind Snapchat+ on older Android OS versions, and is now offered worldwide. If the Styling cache doesn't load up a Night Theme palette, reopen and re-close the APK.