You can do this by modifying the style.css file of the theme. The path to the file on your hosting account would be public_html/wp-content/themes/TheProfessional/style.css. You can also modify theme files with the built-in editor in the admin panel.
The code that you need to modify starts on line 157 and looks like this:
ul.nav ul li a {
background: url("images/menu-bullet.png") no-repeat scroll 0 9px rgba(0, 0, 0, 0);
padding: 4px 0 4px 15px;
width: 145px;
}
Delete the part rgba(0, 0, 0, 0) and replace it with #FFFFFF (or just with white), so that the changed code looks like this:
ul.nav ul li a {
background: url("images/menu-bullet.png") no-repeat scroll 0 9px #FFFFFF;
padding: 4px 0 4px 15px;
width: 145px;
}
Then clear the browser cache and refresh the frontend of your site.
Hope this helps.