The only thing that I can think of is that you made the changes in the wrong place or entered the wrong code. The code I supplied would only change the paragraph spacing and definitely would not affect bolding text.
- Open preferences => Look & feel => Book details
- Copy and paste the text in the code box in my original message into the Text styling box. I chose to paste it between two of the body entries.
- Hit apply
- Check a couple of book details to see if the paragraph spacing has been reduced.
You might also want to hit the Restore defaults buttons before trying again.