Hi @Hel75
Try > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too.
If this does not help then check your purchasing preference information if the theme is a paid for version and that the theme is compatible for your phone.
I wish you all the best. 
