Light theme: it’s the usual white theme that you have always used in these years. Dark theme: it’s the one that you wished to have in WhatsApp! System default: WhatsApp identifies the theme used by your Android system (if light or dark), and sets it in WhatsApp automatically. It’s useful if you use the latest Android version (Android Q) that supports Dark theme system-wide. Set by Battery Saver: available on Android 9 and lower versions, allows to automatically switch to light/dark theme, in according to your Battery Saver settings.
Via: WABetaInfo