Alright, great.
Now, next on my agenda, would be the background and fog. Firstly I'd want you to go to Scenario > Modify Map Size and Camera Bounds, tick the "Modify Map Bounds" in the bottom left corner and then double the size of the map, or something similar. Then go to Scenario > Options and find the box in the middle of the window called "Use Terrain Fog."
Enable that and set the "Z Start" value to 0, then fool around with the "Z End" value between 2000-8000 to see the effect it'll have on your terrain.
After that, or even before that, you can click the "Set Color" button, where you'll be able to change the color of the fog, you can even customize the fog completely by choosing "Custom" at the top and then manually fool around with the RGB values.
That all done, I'd want you to use the added size of the map to add "mountains" to the horizon, so go to the edge of your map and use the raise tool to make your mountains, you can also for better effect have several "layers" of mountains, meaning you put some mountains closer to the camera and some further away. By doing that, and using the "Z End" value of the fog settings proper you can actually make a soothing "fade" effect with the mountains and horizon. Oh, and, lastly, for any of the background to have any effect, you'll have to push back the cloudbox, this is done by holding down CTRL while scrolling with your mouse-wheel.