[Theme: twentyfifteen] Change mobile menu button

Hello! Several of my users have found the mobile menu button (referred to in various forums as “the hamburger,” “the sandwich,” or “the 3-line menu icon”) confusing. As such, I would like to replace it with the word Menu or add that word next to it.

I created a child theme and it’s working fine. I followed instructions to add text (“Menu”) next to the button in this forum by modifying style.css in the child theme, but it is not working:

http://presscustomizr.com/snippet/add-menu-text-3-bar-menu-button/

I realize that the above is from a different theme than twentyfifteen, but it seems that several other forums link to it, so it seemed worth a try. The specific suggestion is to add the following CSS:

/* START OF Add "MENU" text to 3-BAR Menu button  */
.btn-navbar {
width:          90px;
height:         30px;
vertical-align: top;
}

.btn-navbar:after {
content:        "Menu";
float:          right;
margin:         -16px 0 0 0;
}
/* END OF Add "MENU" text to 3-BAR Menu button  */

The site in question is https://aacncontinuingeducation.org.

I would appreciate any suggestions for making this tweak.

TIA,
Rob