By default, the Samsung keyboard is set to a light design, but if you wish, you can also activate a dark design, which means that the keyboard is predominantly changed to dark colours. Alternatively, you can also activate the "Adaptive Theme", which automatically adjusts the keyboard to the colours of the respective app screen.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  General management
Step 3:  Tap on  Language and input
Step 4:  Tap on  On-screen keyboard
Step 5:  Tap on  Samsung Keyboard
Step 6:  Tap on  Style and layout
Step 7:  Tap on  Keyboard themes
Step 8:  Choose desired  theme
  1. Open the Settings
  2. Tap on General management
  3. Tap on Language and input
  4. Tap on On-screen keyboard
  5. Tap on Samsung Keyboard
  6. Tap on Style and layout
  7. Tap on Keyboard themes
  8. Choose desired theme

Samsung Instructions