Lacking explanation of working with custom themes

[joelsgp]

I tried to run this with a custom theme (from this: https://github.com/vitusventure/5thSRD) and got a lot of WARNING - Article missing errors.

It would be good to have a section in the readme explaining how this works/doesn't work with custom themes. Feature-wise, is there a way to make it so that the pdf generation just ignores the custom theme while the html still uses it?