sidescroll custom css

I would like to add a mm or 2 to the borders between the images in the sidescroll – can see here: http://zanchit.co.uk/children-mixed-scroll/ that the images are quite close to each other and feel a bit crowded. Is this possible?

Also would like to do custom css for the scroll bar so that it is one of the colours that I use on my site rather than white. Can anyone advise how I do that?

Thanks so much