In the Field dialog box click the Formula. Click in a cell that should contain the sum of the rows. Click this arrow to reveal a list of visual options including whether to center left-justify or right-justify the equation.
You can also highlight the text in the equation and alter the font size and style as usual.
You can also highlight the text in the equation and alter the font size and style as usual. To fix this you can change the location of the formula or change the formula syntax. On the right half of the Insert tab are some further formatting controls you should be aware of. If necessary click into the Formula field and enter the desired formula.