I want to style the Posts and Comment RSS Icons in the header as they are a dull grey.. but I can't find where to modify the styling???

They are images... You would have to edit the images.

Yes, thanks, I realise that, but I can't see where the CSS is that specifies the styling - the image changes to orange onmouseover (or hover in css) but I can't find anywhere in the configuration to edit it.

There isn't an option. You can code a css override if you want. If you use Firebug with firefox you can go in and look at the CSS that changes it and then create your override.