Example of problem

-apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"I assume If I change Font size for tiddler body I need to change Line Height for Tiddler Body.
I am not sure how px sizes replates to print size.

However as a result of issues when printing tiddlers to PDF I am interested at looking to change the font settings.
... have a closer look at your complete setup ...
Including fonts into TW could be seen as a "redistribution" which is bound to the font-license.
... as a result of issues when printing tiddlers to PDF I am interested at looking to change the font settings.
- Regardless of Default TiddlyWiki fonts, Both Chrome and FireFox result in the browser inspect as using verdana (Proven with inspect tools). I don't see how to change or influence this. This always ends up in the Print to PDF.
Mario,On TiddlyWiki.com using Chrome is reporting the font as the string entered in theme tweeks-apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://www.w3schools.com/w3css/4/w3.css">