RE: is any WCAG criteria to account both default and Dark mode for content

> However, it must not form part of a WCAG audit report – you must report
it separately.

There is no compulsion at all to do two separate reports, so long as we let
the client know in advance what we will be giving them. I provide an
"Accessibility Audit Report" that covers everything in the WCAG plus a
small number of matters of industry best practice and other accessibility
issues. We aren't just testers jumping to call, we are teaching the rest of
the world what is important for disabled people - we are evangelists of
that concept. So I do think we need to wean people away from the idea that
only legal or WCAG compliance matters and the rest can be ignored. Of
course, some clients can and do choose to fix only the WCAG issues and not
the best practice ones that have no WCAG reference, that is their
prerogative. But we should not do that, it is not the best solution for
people with impairments and disabilities.