Something happened earlier is after importing an example flow , the theme changed and even after deleting all the flows , deleting all the unused node ,deleting the cash browser , nothing changed , still having the same problem.
I think the source of the problem is the theme is still stored somewhere even when adding or modifying it. Also the background color of the template node is changed to white according to this line. ( I didn't use it in my flow and seems it was added )
What exactly happened is :
All the background of nodes templates changed to white
The color of title bar changed to blue
No theme modification is possible anymore
I don't see much hope in trawling through the 7000 lines of that file in the hope of spotting something wrong.
You should take this up with the maintainer of the example flow.