I know that we cannot study any Torah which makes us happy on tisha bav- which is generally any Torah (source needed). Based on that, is it generally accepted among (ashkenazi) poskim that studying about the Beis Hamikdash is allowed, since it reminds us that the Beis Hamikdash was destroyed? Or is only learning something that is explicitly sad allowed?
One source that you request in your question is chayei adam 135.
Based on what is stated, if you'd be sad by reading how the bais hamikdash once looked in it's splendor as the gemara succah states
Anyone who has not seen the Temple in it's glory has never seen a magnificent building - מי שלא ראה בית המקדש בבנינו לא ראה בנין מפואר מעולם gemara succah 51b
but then you ponder about how it was destroyed and what a loss it was to klal yisroel, you'd be in line with what one is supposed to do on tisha baav.
If you'd read it due to curiosity or appreciation of architecture or another intellectual purpose, then it would be ossur because it would cause happiness one some level or at least distract you from the general mood of the day.