I cannot find where I can customise font/style for Document/Project notes ?
Anyone ?
Look for the Notes font setting in the middle of the Formatting preference pane.
Ok found it. Didn’t expect it there 
I notice that it only works for new notes. The notes on my previous chapters/documents don’t change. This is also why I never spotted this setting I think.
Correct, you’ll note there is a statement in bold at the top that declares that all of the settings in this particular section of the preference pane will only apply to new items. We’ll never wipe out formatting you’ve already created (mainly because there isn’t a realistic way for us to determine whether that formatting was something you spent hours manually crafting, or if it is just something generated by the software as a default).