2 Wait for Twilight
Don’t take pictures at night. Generally, even dedicated cameras struggle taking good pictures of Christmas lights when it gets too dark outside. Instead, the professionals go out at twilight and start taking shots when it’s dark, but there’s still some ambient light. This helps the camera to capture the beautiful illumination of Christmas lights without everything getting lost in the shadows.
The same is true of your iPhone app! So when it starts to get dark, make sure the Christmas lights are on, and begin snapping photos. As twilight cycles to night, you should be able to find an ideal balance between the dark background and your photo clarity.