The problem is that settings from the Windows 7 theme you are using carry over to Wordpads text background.
Solution is either to set theme back to a default Windows theme or change current theme settings as described below. Before changing the color settings, make sure to write down the current RGB values so you can change back to theme color if the change is unacceptable. Find the current RGB colors for current theme, click on the color boxes mentioned below in instructions but don't select a new color. In the color options window that opens select 'other'. In the new window that opens write down the red,blue & green values for current theme colors. Now you can navigate to and change settings as follows.
In Windows 7, right click desktop and select 'Personalize'. In the window that opens select the 'Window Color' option icon at the bottom. In the resulting window select 'Advanced appearance settings...'
Now, in the new window that opens go to the drop down menu under 'Item' and select 'Window.' There are two colors you can change here. Color1 is the one giving you the grey Wordpad background. The 'Color' one will be font color. Keep in mind this affects other aspects of system and not just Wordpad so make sure to adjust both. For instance if your current settings are a grey background and a white font, if you change background to white but leave font color, you won't be able to see any text as colors will be same. You can try using different shades of grey for font color to keep theme looking OK. Once you have changed colors, hit apply/OK. Change is immediate.