You should probably start your own thread about winter board for more, specific views and help…
That being said,
The iPhone 6, and 6+ have and use different file names for images.
So old themes will not work.
Here's an example,
The iPhone 3GS has an image: image.png
The same image on an iPhone 4 is
Image@2x.png
On the iPhone 6 that same image is
image@3x.png
So the theme you downloaded might have an image, but it might only be theming image.png and
image@2x.png and so on and so on.
Also sometimes files change names and or location in different ios's, so
1. Make sure your getting a newer theme that will work on your sized device and
2. See if some elements of the theme work. Then you will know if it's only because resources have changed names/locations…