How do I make the navigation links bold on hover and active?

Hi there,

you can use this CSS:

.main-nav > ul > li:hover > a, .main-nav > ul > li[class*="current-menu-"] > a {
    font-weight: 500;
}

The only thing with doing that is you will notice movement in the menu items as the font expands.... you may be better off just using a lighter color for the menu items and make it darker on hover and current via the Customizer.