Any embedded images that are larger than 690px wide will be embedded as a thumbnail, and will trigger a lightbox if clicked in a post (that has a dark background #0b0b0b).
While I understand your desire to have a dark overall background on the entire site, I feel the dark on white is more legible/readable. (Coming from a guy whose personal blog looks like this… 
).
If discourse upstream offers a user-defined option to implement different themes, I’ll gladly look into it!