Creating seperate main & page backgrounds

@Sheree: You can add the following css in “Appearance => Theme Options => Custom CSS” box to add white background color behind the content.
#main { background-color: #fff; }

@Sheree: For spacing, you can add the following css in “Appearance => Theme Options => Custom CSS” box.

@media screen and (min-width: 961px) {
#primary {
    padding-left: 2%;
    width: 63.64%;
}
#secondary {
    width: 29.28%;
}
}
@media screen and (max-width: 960px) {
#primary, #secondary, .contact #primary, .contact #secondary {
    width: 96%;
    padding: 0 2%;
}
}

For the Slider image, you can use “Featured Image Slider” and upload the image of your own choice. so, first you can crop the image as per your need using photo editing software like Photoshop, Photo Editor and so on. Then upload it.

@Sheree: You can remove the slider effect from “Appearance => Theme Options => Featured Slider => Slider Options” and then Disable Slider Background Effect.