instead of define class in custom.css, is there any other easier way to change colour?
custom.css file is the best option, otherwise you will need to apply new customizations each time you will update your template version.
how to change the colour of search icon
Add to your custom.css this
.fa-search:before {
color:#yourcolor;
}
and its text field border colour ?
add to custom.css file
.nav-search .dropdown-menu .t3-module .search input {
border-color:#yourcolor!important;
}
how to change colour of the fa fa map maker in breadcrumb ?
add to your custom.css file this line of code
.fa-map-marker:before {
color:#yourcolor;
}