Rick does not cover Thessaloniki in his books. If you do a search, you'll find some old threads about it. To search on a desktop computer, look for the magnifying glass to the right of the dark blue "Rick Steves Europe" near the top of the page. To search on a tablet or phone, click on the horizontal lines at the top left; search is on top, over "About Rick."
Here's what I got when I put "Thessaloniki" into the search window and restricted results to Forums: https://search.ricksteves.com/?button=&filter=Travel+Forum&query=thessaloniki&utf8=%E2%9C%93
Here's one of the more recent threads (well, less ancient anyway - it's from 2015) with some tips that may be useful for you: https://community.ricksteves.com/travel-forum/greece/thessaloniki-where-to-stay-and-what-to-do
Laura, one of Rick's staff, has been to Thessaloniki. If she doesn't respond to this thread, send her a private message using the link at the bottom of this page: https://community.ricksteves.com/users/86530