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Let’s see a page with this menu

I believe you, but I can’t provide code without seeing a Webpage.

If you’re not using a Child Theme, and if the theme doesn’t have a designated section of the dashboard for CSS modifications then install this Custom CSS Manager plugin http://wordpress.org/plugins/custom-css-manager-plugin

Then use its “CSS Code” section of the dashboard to hold your CSS modifications:

Alternatively use your Child Theme style.css file to hold your CSS modifications:

#nav-header .nav-text {
    display: none;
}
.nav-toggle {
    float: none;
    margin: 0 auto;
}