You can only set JavaScript menu styles after the JavaScript menu parameters have been set at least once. After the JavaScipt menu parameters have been set the tool will be available as shown below:
You may set fonts, colors, sizes, background colors, etc. as desired for the top level, and for two sub levels. Note: your menu may contain up to 5 sub levels, but all subsequent sub levels will inherit the styles for the second sub level.
Click on the #RRGGBB links next to the color entry boxes to bring up a color picker. Click Update to save your changes.