After inspecting the HTML for an Export to Simple HTML page, I expect that customizing the CSS is only part of the solution.
As I imagine tuning the page layout, I know I'd want to affect the layout beyond just the CSS, e.g., moving the generated date to the top of the page.
The customized invoice templates in IGG Software's
iBiz 3 would be a good model to follow. IGG offers a
collection of templates, a few of their own plus many user contributions.
[submitted as formal feedback]