There is not as much custoscene I think for 8, because the default theme is so damn good for once. It already is sleek and minimalistic style and you can make it pretty much any color. It probably takes care of style complaints for most people that hated Fisherprice or glass stuff in the past.
I've been into Windows custo for like a decade at least and there is barely anything I do for change 8 besides some icons replacement/removal. I like the 'metro' style icons for everything, and always have to get rid of those awful 'film' overlay icons for video files. I might have a go at tweaking around the msstyle a bit to move some things around (want to move stuff like the menu and ribbon collapse buttons up into the titlebar) and 'fix' the file preview pane. Pretty simple mods really.
Vista, 7 and 8 msstyles take a LOT more work to do than XP so there have been less for that reason. Also MS likes to change how things work, where resources are kept in each release a little and it sometimes takes awhile for everything to be relearned and discover limitations.
Eh, while the default theme in 8 isn't really bad, that's not always the main reason people customize windows. For many it's just about variation or making it more personal.
I think the reason for the reduced scene is because a lot of the members have sorta grown past it and have other things that take up their time and attention, and as you say; there's more work to be done than there was on XP, so there's not a whole lot of new blood.
edit: also, quite a few have moved over to either Linux or OSX over the years as well
If you go on deviantART and look you will see quite a few new custom styles, but the majority of the stuff that gets released there is either downright terrible or completely unoriginal at best. If you're lucky you may catch one of the good releases though.
Remember themes in XP? They had to be signed by Microsoft, and they only ever signed like about 3.
So hacks appeared for each service pack level to allow custom themes to be installed.
Thousands of those abound!
It's still like that; to use custom themes you still need to mess with UXTheme.