If the horizon line is coming in as a paper thin white line across the background that can be really annoying. There are not many things we can do but there are a couple.

1. In the Skylab, go the the Clouds Cover. You will see a Cloud Height slider, make sure this is set to 100% maximum for Cumulus. There is a special relationship between the cloud height settings and the altitude and smoothness of the background Haze gradient. The Cloud Height setting affects the haze even if the Clouds themsleves are not activated, so this is a really important trick to keep in mind.

I find that the taller and smoother the horizon gradient the better the results because the transition from blue to white isnt nearly so sharp.

I have some other tricks you could try as well but try this one out first, if it doest get the job done then please report back and I'll give you more directives. Best of luck!