EagleFiler always opens RTF files using the colors that are specified in them. However, you can set the text and background colors to white-on-black if you want. (Note that there are separate buttons for setting the background color of a run of text vs. the whole document.) Once you have a file set up the way you want, you can save it as stationery so that new documents can easily be created with those styles. You can also set up a named style for white text to easily apply it.