Alright, I've found that setting textShadowAlpha
helps:
s|ActionBar, s|ButtonBar {
chromeColor: #0066CC;
color: #FFFFFF;
titleAlign: center;
textShadowAlpha: 0;
}
And there is also textShadowColor
available and used by mobile/ActionBarSkin.as, mobile/ButtonSkin.as and mobile/supportClasses/ButtonSkinBase.as