I just spent hours digging through engine settings and found a workaround!! I had a ton of darker assets and literally could not kitbash my environment together…
In your Content Browser settings enable “Show Engine Content”
Scroll down to the Content Browser Engine folder, search for both FloorPlaneMaterial and SkySphereMaterial
When you open each of these you can manually adjust the Base Color. The FloorPlane will be most of your thumbnail, the SkySphere is the top little bit of the thumbnail. You can set these however you’d like but FloorPlane 0.1 and SkySphere 0.12 is a pretty decent medium grey.
Its not perfect but omg i can see my assets again!!