I would really like to use theme 6 or theme 3, but every time I try to put in the code, it's just white and black. How do I get the colours to look exactly the way they do in the live preview?
I guess it’s because tumblr tends to take the colours from the theme you used before and ignore the ones that the theme maker defined.
Try the Reset Default button underneath the Appearance tab (where you edit the colours and links etc). Usually this doesn’t work (for whatever reason), but save it anyway and then refresh the page. You can also try refreshing, saving and closing the customize page, and then go back to edit the colours.