The weight of my font seems to change once the app is built. It originally displays slightly heavier and then you can see it change a sec or so after it loads. I don't know if I messed something up in the css. I'm using arial if that helps.
The font change you are describing is a common thing that occurs when content is loading on a device in the container app. I use pubcoder to build ebooks for patient info in a hospital and I get around this by having the text fade in on loading. Hope that is of some help.
Angelo might have a better fix.
I also thought i would mention that i use shelf and think its pretty cool.
What do you use pubcoder for?