If you're using Hotel Master theme, please read this : https://goodlayers.ticksy.com/article/12190/ 

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TourPackage Theme - Creating 'Boxed Layout'!

Try to add this css in additional stylesheet in admin panel.

div.body-wrapper {
width: 1020px;
margin: auto;
background: #FFF;
}
body {
background: #555; 
/* background: url('IMG_URL'); */
}

Hope this help :)