I double checked and I think the highlighting of a selected menu item is not working correctly in the MegaMenu.
The images below show that the highlighted item is shown correctly using the online T4 Demo, but it shows ALL submenus as being selected on my site with the updated code.
I will continue to check, but I don't think I altered any CSS for megamenu classes.
Dave
Hi Dave,
It's a style issue of megamenu, it's fixed now, kindly check.
Thanks saguaros,
What did you change ?
So that I can update my offline verison also. Can I use my custom colour pallette on the mega menu now ?
Dave
OK, I can use my own pallette - just checked. 
What file did you change so I can update my offline version ?
Thanks,
Dave
Hi Dave,
I edited this file: plugins/system/t4/themes/base/css/tpl/pattern.tpl.css
.__class .nav > li.active > a,
.__class .nav > li.active > a.nav-link
.__class .nav > li.active:hover > a,
.__class .nav > li.active:hover > a.nav-link {
color: __link_hover_color;
}