Word of caution…never edit the theme’s own style.css file because you lose your changes whenever a theme update comes along.
Using a plugin like Simple Custom CSS is what you would use. This adds a “Custom CSS” link under the admin menu group “Appearance”. This is where you would create or add custom CSS.
Regarding your menu items and how the menu wraps to a second line, this is currently due to the long menu item titles for each one, so they take up more space. The only way to get it back into one line is to make the menu font smaller. Current styling is:
.primary-navigation {
font-family: "Playfair Display", Georgia, serif;
font-size: 1.125rem;
font-weight: 400;
font-style: italic;
}
You will want to make the font-size to something like:
.primary-navigation {
font-size: 0.875rem;
}
You can also adjust the padding on each one which is currently using:
@media (min-width: 992px) {
.primary-navigation li a {
display: inline-block;
padding: 0 0.5rem;
white-space: nowrap;
text-decoration: none;
color: #222;
}
}
So your change would be:
@media (min-width: 992px) {
.primary-navigation li a {
padding: 0 0.375rem;
}
}
I wouldn’t recommend going less than that, otherwise your menu items will be too close to each other.