Xerte 3.12 inspects the theme.info file to see how to cope with buttons (are they images or font awesome icons). That does not work with offline exports, because the info file cannot be loaded dynamically.
Some of the standard css files have been moved to the theme/Nottingham,/default folder. Those rae sometimes used by the other themes. Make sure the default thme is always included in an export (like the colourchanger themes)