When Im pondering a new theme I make a few lists. I make a list for several catagories
Elements- these are things that should be found in the envoirnment I want to create. Both things that set the mood, place and time period. In a gold mine I expect to see
old lanterns, Bowie knives, those pans, trough system for panning, mining cars, pick axes, rope, shovels. I back up as much as I can and look for things that will cement the reality Im creating. I would look at the year 1849 (or when you want) and see who was president campain posters really help set a time and they can go on a window. If you have an office then make a fake calander with the date you want.
another list I make is of sets that fit the theme. Obviously you would use the best ideas from each list. A good list of sets might start like
Mine office, Mine shaft, blacksmith area (to repair/make tools) you could rig a spark effect in there, saloon, a crystal type cavern could be cool, a cavern full of bats. Think of as many as you can and then you have alot of good stuff to draw from.
the last list I make is of characters that would fit, prospectors and such.
Im off to bed, good luck with it,