Fix BL-744: Javascript error the first time a user modified a style

Our previous code created the necessary element in <head>, but in the
xulrunner 29 we're using, the element we added isn't a Stylesheet
object. So this fix gets the new stylesheet object from the
document.styleSheets array after creating that new element,
and then all is well.
Merge pull request #175 from BloomBooks/BL-744ErrorOnFirstStyleChange

Fix BL-744: Javascript error the first time a user modified a style