The standard UIQ3 programs like Contacts, Messages and so on do not offer much flexibility regarding fonts. They offer zoom levels which then change the font size, but that's about it. You can see also that fonts probably were not very high on the list of priorities by noting that UIQ3 phones like the P1i and the P990 only have one single font in firmware. If you want more, you have to install it yourself! (More about this in chapter UIQ3 Fonts.).
MoStacks on the other hand is very flexible regarding fonts. Even if you are completely content with the readable, sans-serif built-in font, you can still use that flexibility to configure exact point sizes (not just 3 zoom levels) to make important fields large, or make lists to use a small font so that they show a lot of data in one glance (even if readability will decrease a bit, as a trade-off).
Fonts in MoStacks work along the four-level hierarchy of stack, backgrounds, cards and fields, with hierarchical inheritance.
If nothing in your stack carries specific font information and you switch the stack font, almost anything in the stack will switch its font, because of the hierarchical inheritance. (This has about the same effect as standard UIQ3 zoom levels, and because of this the implementation of "true" zoom was given low priority, and in the current version it's missing.)
Note that you can specify incomplete font information, because you can specify or leave blank face name, point size, attributes (like bold) and color independently, and the hierarchy plays out for them independently as well.
When MoStacks has to display a field and must decide the exact font to use, it looks from the bottom up the hierarchy, so to speak. For each of the dimensions "face name", "point size", "attributes" and "color" it looks in turn at the field font, the card font, the background font and the stack font and takes the first data that it encounters. If no data whatsoever can be found, a system default of Arial 11 points, plain, black (on Windows) or Sans-Serif Latin 11 points, plain, black (on UIQ3) is used.
For example, if you only configure point sizes and attributes at various places, you can still switch font color at the stack level to switch color for all fields, e.g. to better harmonize with a theme.
Unfortunately it turned out that not all UIQ3 controls fully support font configuration. Some of them stubbornly insist on using whatever font and font size the theme (or some system default) instructs them to use. Among them are the drop-down portion of a combo box, date controls and time controls.