I think I found an oddity with checkboxes too. When used in an imported widget group, the fontcolor seems to change based on the on|off status, tho the docs only list one font color available. Try using a nice bright color to make it more obvious.
This led me to a second issue... I tried setting fontcolor:0 and fontcolor:1 for checkboxes (even tho it's not supported, I was just trying to narrow down the previous issue) in an imported plant, and it led to parsing issues. Here's an example plant that triggers it for me:
checkboxtest.xml (645 Bytes)
If you look at the monolithic file, it becomes a bit of a mess on that line.