It could be done with some custom css. If you're just wanting the text to be bold and white you would need to apply your own css to the css in s5_flex_menu.css for this.
I have played around in the above css but I can not find the line where I control the highlight color when an item has been selected. I would like to have it white and bold.
I would appreciate your help very much as this is the last thing before the release of my site.