Samsung S8 has this "Night Mode" feature, akin to "Dark Theme". Sadly, KDE Connect does not detect the "system default" to be a dark theme, and instead uses the Light Theme. Device has Android 9 version.
System-wide dark theme preference was added in Android 10. What you are seeing is probably a Samsung-specific extension. It's unlikely that we will add code to support a Samsung-specific extension, especially since there already is a proper thing we support