Currently, it is impossible to change the Roblox app theme based on your device settings. In addition, it is impossible to have a different theme on the mobile app than on the website.
The Roblox mobile app should have a third theme option called “Automatic (follows OS name settings)”. When this option is used, the Roblox app theme will change automatically based on the operating system theme.
In addition, it should be optional to sync the appearance theme across clients rather than forcing synchronization.