I hope you are well today and thank you for your question.
The code shared on the other thread is for Sparkling theme and as you have posted the question in this Travelify theme forum then i think you are using Travelify theme so in this case you should use the following CSS code by adding the following CSS code in the Custom CSS option of your theme on the below path.
Admin Area -> Apperance -> Theme Options -> Other -> Custom CSS
#main-nav li a {
text-transform: lowercase;
font-size: 18px;
font-weight: bold;
}
Change the values of CSS properties in the above code to whatever you want to.