A while ago I changed the way the lighting works in the ModelConverterX preview. Before the light would always come from the view point, while now there is a separate light location. The downside of this change is that you now can see that some sides of the model are always darker.
I have been thinking about adding a nice time of day simulation where the sun moves around your object, but in the end I decided that is probably not so useful. Instead I have now added two sliders where you can manually alter the direction where the light comes from. The azimuth slider controls the direction it comes from and the elevation slider determines the height of the light compared to ground level.
You need to click on the button with the three yellow arrows you show or hide the toolbar where the sliders are location.
I hope this feature makes it even easier to inspect your model in the right conditions.