Working on the same sheet all day long and dealing with tons of data can be very exhausting. You may consider bringing some new colors to refresh the tired eyes. Changing the background seems like a good idea.
1. Switch to Page Layout tab and click Background in Page Setup section.
2. Then a window of Insert Pictures will pop out.
3. You can choose From a file to browse the folders on your computer to select the picture you want and then hit Insert to set it as the new background.
You can also type the keyword in the textbox of Bing Image Search to find a picture you like and hit Insert to set it as background of your worksheet.