make sure you’re saving the theme to

~/.emulationstation/n64

also make sure you’re editing both the <basicTheme> section as well as the <theme> section.

if you’re editing the files in the location that they were originally, its probably that you didn’t edit the basicTheme section.

If your theme is totally destroyed, as in a white background, red header, then you probably did something wrong with the XML file completely. Either you didn’t close a tag or your encoding is wrong (more than likely the former instead of the latter)