I suggest you choose the darker generic grey at this point. Once you get a feeling for the results, you may want to modify the base and line colors to suite your taste.
Fill the rectangle with dark grey. Next, select the line tool (smallest line thickness) and choose the generic light grey.Use the zoom key to focus on the upper left hand corner then make a line as shown below (notice the grey line is not right under the top of the rectangle – black line).
