+1 to jazzer. There used to be a plugin in Compiz that just does that. http://wiki.compiz.org/Plugins/Colorfilter
. Too bad Compiz is pretty much dead.
There is an alternative though that I have tested to be working in one of my F16 virtual machines. Attached is the screenshot.
As you can see, it is not just the theme, the whole "frame" is turned to grayscale by X server, including the movie clip.
Unfortunately, the steps that I will outline below may or may not work with your graphic card driver. For this to work, your graphic card driver (kernel module) must be able to support 8-bit color depth. If you are still interested, here are the steps which allows me to switch to grayscale on demand.
1. Download the attached xorg configuration file and then put it into "/etc/X11/xorg.conf.d" folder as root.
2. Start the X server by providing the extra option to override the color depth to 8 bit.
$ startx -- -depth 8
Your mileage may vary, but I can achieve this by booting into multi-user mode (i.e. level 3) and then starting X server manually to add extra option. While you at it, you may want to try the "-retro" option as well to see how X starts in yesteryear. If I boot normally (level 5) then the display goes back to normal because the default depth is 24 for my case.
Personally I don't see much point of doing this but If you really like this that much
then you can set the default depth as 8 bit in the configuration file as below.
Identifier "B&W TV Nosta-gila!"
This will make X server always start in Grayscale by default.