Have a look at the Tool Options. There is a pull down menu (I forget the name FILL?) that has options for Lines, Areas and Lines & Areas. Fill tool uses this to paint the LINES only, the AREA in between lines only or both LINES and AREAS with your choosen color.
If not, I see in your screenshot, that you only have 2 colors (white = background and black=lines colors). Try creating a New Style and give it another color and then try to FILL that color.