Hi, sorry, I dont check by here all that often! (feel free to PM me if you dont see a response here, as at least that tends to email me!)
Wow, that fort map... I dont remember seeing that, I will try to take a look when I get a chance.
For the sky graient colour look in the Settings.txt files in the theme folder, it should look something like:
themeid: 21
waterlv: 0
watercol: 0
skytop: FF404264
skybottom: FF62412C
Some of these are experimental / no longer supported by WA itself, but the themeid number is what dictates the falling objects and gradient colours I think.
Skytop and Skybottom should probably be set to match. (though I cant remember why, something to do with palette generation for the map I think), but if you find another theme that has the right gradient you can just copy it's Settings.txt file across.
As for girders... I have concidered it, but not really dug to deep. If enough people want features I am happy to try.