So I wanted to ask you guys what do you do when you've decided the theme of the map?
To be blunt, a lot of us, may do stylistic changes during the planning and early development phase, and because of that, things may be changed or removed all together. Very few of my maps that are specifically stylized, keep me engaged long enough to see them enter alpha, a good example, (but current issue being compile crashing my pc) PL_Snowed_in, is a industrial/rustic trainyard set in north Dakota during the winter. another is KotH_Navajo (which is currently released, but discontinued production, in Alpha 1 stage), which is set near a hydroelectric dam somewhere in Hispanic south western United States.
How do you decide where the buildings,routes etc are going to be?
In my honest opinion, everything is subject to change, either due to hardware limitations, bad composite (IE, the set up you wanted for the 2 capture point sucks ass, and is unbalanced), or you just think of something better to put their instead. Personally I don't resort to planning, as I feel I can do much better without it. And a bad advice for newbies just getting started, is work out layouts per point. I do this, so I can minimize work and allow for some creative layouts, but also may not be consistent. Conversely, it may be less creative and more work.
In all honesty, a map should be fun (with the snags of crashing, errors, and criticism), but don't fret, the experienced map makers are always looking to help, and all you need to do is ask for a reference, advice, or just an opinion on something you did. I created my own set up Payload track (with switches), and I ran into a lot of errors, bad Ai, bad set ups. And I asked around the community, and the only bugs I have, is I need an end point (reasonable), and my Prop library does not exist anymore.