There may be several steps to this:

1. changed the default in the prefs specs (ie. factory defaults) to use this font
2. check that it is then used. Possibly wx needs informing of the ttf file location. DejaVuSans.ttf is being included in the Standalone packages in the resources location (this can be found using prefs.paths['resources']?)

Ideal additions:
3) it sould be great if changes in prefs were immediately applied to the open documents (currently the code for applying text styles is only run during file load, but it should be moved to a separate method like self.updateStyles() that can be called for each document)
4) if 3) has been done, it would also be great to add a quick keyboard shortcut to expand/shrink font sizes of the coder (default shortcuts might be Ctrl-+/- but determined using the prefs specs as with other shortcuts).