Hi tuhinbiswas98. Here is the theme default css to set the topbar navigation. You could paste this into a child theme style.css file, the theme Custom css options (only in v2.2.2), or a plugin like Simple Custom CSS and adjust the colors there:
/* common */
#nav-topbar .container { padding-left: 15px; }
#nav-topbar .nav { }
#nav-topbar .nav li a { color: #fff; color: rgba(255,255,255,0.7); }
/* level 1 */
#nav-topbar .nav > li { border-right: none; }
#nav-topbar .nav > li > a:hover,
#nav-topbar .nav > li:hover > a { background-color: rgba(0,0,0,0.1); }
#nav-topbar .nav li > a:hover,
#nav-topbar .nav li:hover > a,
#nav-topbar .nav li.current_page_item > a,
#nav-topbar .nav li.current-menu-item > a,
#nav-topbar .nav li.current-menu-ancestor > a,
#nav-topbar .nav li.current-post-parent > a { color: #fff; }
/* level 2 & 3 */
#nav-topbar .nav ul { background: #26272b url(img/opacity-10.png) repeat; }
#nav-topbar .nav ul li { box-shadow: 0 1px 0 rgba(255,255,255,0.06); -webkit-box-shadow: 0 1px 0 rgba(255,255,255,0.06); }
#nav-topbar .nav ul li:last-child { box-shadow: none; -webkit-box-shadow: none; }
This css should change the search icon color:
/* change topbar search icon to red */
#nav-topbar .toggle-search .fa {
color: #f00;
}