As the text tag list doesn't contain something like outline
Available tags:
pos, size, text, color, fontPath, fontSize, alignment, forceUppercase, lineSpacing
Source: https://github.com/Aloshi/EmulationStation/blob/master/THEMES.md#text
you have to use a font with a outline. Just search for a folder called "art" in the root of the theme you want to edit and replace the old font with the new one.
If you don't want to mess around with the XML files, rename the new font like the original one was called.
That's it. If you need more help or a finished solution, tell me.
EDIT: If a change of the font color would fix your problem to you could edit the "main" XML (it isn't called main in most cases), located at root of the theme folder. Edit the color hash in the <color> tag
