feat(content): October 2015 backgrounds

common/script/src/content/backgrounds.js

@@ -21,6 +21,7 @@ let sets = {
   'backgrounds072015': ['dilatory_ruins', 'giant_wave', 'sunken_ship'],
   'backgrounds082015': ['pyramids', 'sunset_savannah', 'twinkly_party_lights'],
   'backgrounds092015': ['market', 'stable', 'tavern'],
+  'backgrounds102015': ['harvest_moon', 'slimy_swamp', 'swarming_darkness']
 };
 
 generateBackgrounds(sets);