Easy!

First, go intro BrawlBox open up your sc_selcharacter.pac or common5.pac and go into:
sc_selcharacter.pac -> MiscData[30] then in MiscData[30], look for the image name: MenSelchrChrNm.### (example: 001 is for Mario.)
Export the image.
Open the image up in your image editing software, and make the whole image pitch black.
Then for the text color, use the color White.
After finishing your editing replace the image you edited. Done.