goodtognci
Hi
Not sure what you mean by default color. as colors can be changed via template layouts options > edit the layout > Main nav part and select different plat.
About adding custom style you can use this code and apply different custo style on the menu items
.t4-megamenu .navbar-nav > li > a, .t4-megamenu .navbar-nav > li > .nav-link {
border: 1px solid;}