Yes you can make your home screen and lock screen full dark. first go into display setting and choose dark theme. Then go into themes and select any dark wallpaper and install for both home and lock screen. Choose your icon pack as per your liking. That's it.
supposedly you want t o revert back go back into settings into themes and select my stuff and you will find all saved wallpaper
Guys thanks for replies. But I had full dark homescreen, lockscreen before I poked around sittings , wallpaper. I got this Note 10 Lite and it was on One UI 2.1 that time I changed to dark mode. Now I am seriously not understanding how I can't revert back to it.
I didn't set any specific wallpaper before when I was full dark.
It's really annoying and don't know how it happened.
May be someone who has Note 10 Lite on One UI 2.1 can help in understanding.
Yes , I had done it. As I have written in above comment issue is solved with high contrast theme. But I am still not able to understand how I was having all dark homescreen and lockscreen without high contrast theme before that ??? That's puzzle.