It looks like Atlassian gave Trello a new font. Depending on whether you use macOS or Windows, default Trello will now the OSs system font (San Francisco or Segoe UI). What's the thinking behind this? I don't know...
What I do know, is I have very busy Trello boards, and Segoe UI is less readable to me for that amount of data, than a very reductionist font like the original Helvetica or Arial. So, TrelloX reverts this change in Trello, to give you back what I consider a more appropriate font.
I use Windows though—If you would prefer the Apple system font San Francisco on macOS, please let me know 😊.
Also a few other visual fixes, to ensure TrelloX matches Atlassian's UI fiddling.