This is a weird one. Here is Theme Options on site one (trademarkfilms.co.uk):
And here is Theme Options on site two (manifest-nirvana.com):
Same browser, same version of X and Cornerstone. Looks like site two is rendering correctly, the first one looks wrong to me. It has the same rendering issue on the Global Blocks and Cornerstone pages, everything is in ultra-bold blocky text.
Site one was updated from 6.5.6, site two is a completely fresh install. Does this point to a potential problem with the X install on site one?