WordPad does not have the option of changing the background colour it actually reflects the Windows colour scheme. If you find say Notepad with a black background you need to change the colour of windows background
Right click on the desktop and select "Personalise"
Click on "Windows Colour Custom "
Click on "Advanced appearance settings"
You will see underneath "Item" "Desktop" in a drop down menu box
Click the drop down and scroll to the bottom where you will see "Window"
If "Colour 1" is set to black change it to white or whatever is your preferred colour