thank for sharing
for anyone can not import theme
you can try this way, this work for me on ue5.3
"ALSO - When I tried using the ‘Import/Export Theme’ buttons they both fail.
I needed to place themes in C:\Program Files\Epic Games\UE_5.1\Engine\Content\Slate\Themes\ to see them. (I had to create the ‘Themes’ folder in that path)"