>>10
Since the background color on the post you're peeking at is hardcoded there are some styles that are not 100% useable, like Blue Moon has the same text color and background color.
I think this could be solved by wrapping the post in div#posts div.thread div.allreplies, and then removing the border and margin on .thread with style attribute to make it look like a normal post. This way it would use the background of the selected theme. Adding multiple #posts id isn't really valid html, but afaik nothing on Wakaba's javascript relies on #posts so it's just unfortunate the dev used id instead of a class there, but not really an issue.
The much easier temporary fix would be to just hardcode in the text color as well so none of the styles make it unreadable.
All in all pretty useful script though.