Here are the two ways I could think of:
One of them is done in the settings page tab. Click edit the page you want to change and open the html box. At the top put <div style="color:white;"> and at the very bottom put </div>
The problem with this method is that you will need to do this for every page and the text in your preview window will also turn white so you won't be able to see what you type.
The other way is to climb into the customize html page code. There you need to search for the <div class="page set-right"> code.
Just change it to <div class="page set-right" style="color:white;"> and that should change the text on all the pages.