How do I change the grey backgorund on each page to white?

If there is no option to change this in your theme settings then just change the CSS.
Go to your template.css file (http://originalnutterdesign.co.uk/wp-content/themes/dw-fixelnew/assets/css/template.css?ver=3.8.1)
Search for the following phase

.page .single-page .main-content {
background-color: #F5F5F5;
font-size: 15px;
padding: 30px 30px 0px;

Change it to

.page .single-page .main-content {
background-color: #FFFFFF;
font-size: 15px;
padding: 30px 30px 0px;

Save the file, hard refresh the page. If you use any caching plugin or CDN, Don’t forget to flush it.
Hope this helps.
Anik

add this in your style.css
.page .single-page .main-content {
    padding: 30px 30px 0px;
    font-size: 15px;
    background-color: #ffffff;
}