You Will Be Able To Change The Theme of HTC 10
Even though you can customize your Android phone with different launchers that you can get from Google Play or from the internet, you are limited with the stock launcher if you liked it more than others. If the stock Ui does not have any option to change the theme, again you are limited.
But, new screenshot, probably from HTC 10, shows that you will be able to change the theme of the stock launcher. It is not something new since it has also used in recent Samsung phones, but having more options to customize the stock launcher is something nice.