Hi tuhinbiswas98. Following is the default theme css for menu colors:
/* common */
#nav-header .nav li a { color: #fff; color: rgba(255,255,255,0.7); }
/* level 1 */
#nav-header .nav li > a:hover,
#nav-header .nav li:hover > a,
#nav-header .nav li.current_page_item > a,
#nav-header .nav li.current-menu-item > a,
#nav-header .nav li.current-menu-ancestor > a,
#nav-header .nav li.current-post-parent > a { color: #fff; }
Changing the common code to this should make the top-level menu items a dark grey:
#nav-header .nav li a { color: #000; color: rgba(0,0,0,0.7); }
Changing the second color line will make them turn black on hover:
#nav-header .nav li.current-post-parent > a { color: #000; }
If you want them to be black all the time, remove the rgba style:
#nav-header .nav li a { color: #000; }
Here are a couple of tutorials on fonts:
https://wpsites.net/wordpress-tips/free-google-web-fonts-for-wordpress/
https://premium.wpmudev.org/blog/custom-google-fonts/
https://premium.wpmudev.org/blog/custom-fonts-css/