This I believe answers your original question.
I reckon I read your question right, here's the real answer...
On my OEM Samsung S8+ running Android 9.0, which has had no network provider tinkering of any kind, just current updates as Samsung intended.
Here you'll find all the things you can do with the Standard Samsung Keyboard including change theme colour. 7 Easy to follow steps below...
1> Swipe down from the top of the home screen
2> tap on the settings cog icon
3> select General Management
4> select Language and Input
5> select On-Screen Keyboard
6> Select Samsung Keyboard
7> Select Keyboard Layout and Feedback