How to bold main categories?

Hi!<br>
I have a WP blog. I have this template https://themes.wordpress.net/columns/2-columns/3323/green-valley-10/
How can I bold my main categories (parent categories)?
<br>
I’ve tried to add this code to style.css:

/* BEGIN NAVIGATION
----------------------------------------------------------- */
#categories ul {
list-style: none;
}

#categories ul li {
font-weight: bold;
}

but it doesn't work :(
<br>
Please help me.