As this is hard-coded into the shadow function...after all why would you want to theme it... 
Line 190-192 of shadow.js:
Code:
if (Ext.supports.CSS3BoxShadow) {
targetStyle[this.boxShadowProperty] = '0 0 ' + this.offset + 'px #888';
}
It would appear that you will have to put some nice css code with !important to get around this. Which I am sure you agree is horrible, but looking at EXT's css they use it everywhere, so it must be ok. [I hope you all know that I am joking and that ext should invest in writing correct CSS rather than throwing !important around every 5 seconds...]