Hello, Luis,
thanks for your interest. I see I have to clarify the issue some more: I use the app DevonThink Pro. I have it newly installed on my fresh installation of El Capitan on my SSD-drive. After starting the app up for the first time I realized a. that the font size was too small for my liking and b. it was not happy with the font that was chosen by app/system, the infamous "system Font". It was just there, top of the list, activated/chosen automatically. I switched to another font but then thought: hey, that San Francisco font might look better if I'd use a bigger font size.
But I could not get my change reverted. The "System Font” choice has since disapperead from the font selection menu.
So there you have my question: how can I get the menu back to letting me choose that font?
Thanks again!