Other than adding a color to the page background in the word file, you can also add an image background with the steps below:
Step 1: Click the "Design" tab from the ribbon;
Step 2: Click the "Page Color" command in the "Page Background" section;
Step 3: Click "Fill Effects" from the drop-down list;
Step 4: In the "Fill Effects" window, click the "Picture" tab;
Step 5: Click "Select Picture" in the "Picture" tab, and navigate to the picture location or use the online picture;
Step 6: Click "OK" to finish and you now have an image page background.