After 14 days the phone theme will revert to stock from the third party free themes. You are only allowed to keep premium themes, which Samsung gets a cut of. Their excuse? Fund our designers only.
https://www.theverge.com/2018/11/15/18097179/samsung-free-themes-14-day-limit-galaxy-one-ui
https://www.theverge.com/2018/11/15/18097179/samsung-free-themes-14-day-limit-galaxy-one-ui
Samsung is headed towards a controversial change that impacts customers who like customizing their Galaxy smartphones with third-party themes. The company has begun sending a notification to users that warns beginning with Android 9 Pie, which Samsung plans to launch in January, it will only permit free themes to be used for 14 days. After that, the phone will automatically revert back to Samsung's stock theme. The Verge has confirmed the notification firsthand, after Droid Life and SamMobile reported on it.
Users will receive two pop-up notifications before their free theme is removed; the first will be displayed 24 hours before the 14-day expiration. The second will hit 10 minutes prior to the cutoff. Samsung says it'll "provide suggested themes along with the notifications in order to help you easily change your theme." Presumably, those suggestions will be for premium, paid themes; Samsung gets a cut of those transactions from its Galaxy Store, whereas the company makes nothing from free themes.
Samsung's reasoning for the shift is pretty questionable, to say the least. "We ask for your understanding as we have changed the policy in order to help our designers continue to create high quality products and also to provide stable and satisfactory services for you."