Make print preview unaffected by theme-related changes to email content

When designing a theme, especially a dark theme, it might be necessary to modify the appearance of email contents, via the .ignore-in-parent-frame specifier in email-frame.less. One example is the default text color. These changes to the email content now also apply to the print preview, which is not desirable.

Some workarounds are possible

  • Using different css specificity for normal email frames and email frames in print previews (although this is tedious with the available options in terms of css specifiers).
  • Using the @media print query in print preview window. However, this query is currently ignored in the print preview window, while it is applied in the actual print result.

I think it would be better to make the print preview entirely indepent of any additional css markup in the email frame.

(Originally posted by asparc on GitHub.)