hello . i use win 10 too but i dont have problem for opening themes . going file > open and it work fine .
about testing theme . using "use visual style " after saving theme cause black screen in win 10 dont have idea why . tried it once for and ended up re installing windows .
for creating theme . i suggest you try this links for maps . actually this are for win 7 but work on 10 too , some of maps are changed in 10 but you will find them .
http://virtualcustoms.net/showthread.php/28810-Complete-Guide-to-Theme-Buildinghttp://vistastylebuilder.com/forum/index.php?topic=1179.0sry for bad english . good luck