If you are looking for a way to create the new white thumbnails, go to Photoroom Light! And if you want the dark thumbnail style, use the one included in the game.
Originally inspired by Defib's lightroom, I found that there weren't any thumbnail rooms that you could take thumbnails that look like original BeamNG ones.
This time you can put the quality up to the max for nice shadows and reflections, so here are some updated screenshots:
Fog blend level could be a tad higher, but it isn't noticable unless comparing between official and custom thumbnails. But it certainly looks much better than some thumbnails I've seen: