Highlight active menu item doesn't work

The link text is wrapped in a span tag (with class menu-text) which gets a color value from the theme CSS, meaning that it overwrites the a tag color you specify. I also don’t think you need the .act class in your selector as it won’t highlight custom menu links, so try the following:

.main-nav > li > a.mPS2id-highlight .menu-text {
    color: #fff;
}