It is easily overlooked, although the reason it’s situated here is to put the CSS editor inline with the invoice allowing you to view live changes to any document you’re previewing. Having it here allows you to more effectively apply different styles and see the output immediately without jumping between screens and refreshing.
This feature also came much later… perhaps we will eventually merge this into one area (which does make more sense) it’s just a hefty job and it’s one of those areas you don’t need to go into too often so it’s hard to prioritise it over other feature requests right now.
@Peter_Watson - the invoice gallery option is on the invoice preview rather than invoice customisation. When you preview an invoice, the option will appear in the top left hand corner in agreement box. Please see my screenshot above.
If you need any help with the editor, then please don’t hesitate to ask