PDF preview should not be affected by a dark theme in Thunderbird

[Terex83]

I'm a long-time Thunderbird user and receive emails with pdf attachments frequenty. I also love the dark theme, but I cannot read the pdf document in Thunderbird because they too get inverted into a dark document without colour, making them hard to read. A simple code change can prevent pdf documents from changing appearance in Thunderbird with a dark theme applied. 

I would be so grateful if this is implemented. For now, I open pdf attachments using an external pdf viewer.

[Jon]

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