how i can change my theme in htc 826?

[AC Question]

I updated my android version lollipop to marshmallow and then there is htc sense 7.0 is also updated but i dont apply any themes on my mobile...id i download theme from htc sense theme application it still showing me downloading for long time and in notification it shows download 100% ....plz help me if any one knows how to download and apply theme in htc 826 marshmallow