Don't worry, the dirt will turn to grass eventually.
In front of the portal, over the stairs, I managed a decent design for the ceiling.
Here's what it looks like as you ascend the stairs.
The smooth stone is a bit of a deviation from my general pattern, but it adds depth to the build. I bet a lot of time will be spent going through the base looking for ways to make it look even nicer. I recently found a mesa biome, so I could perhaps color code portions of the base.
Finally, a view from the portal itself.