Appearance → Customize → Typography → Navigation font. Five fields: face, size, weight, letter spacing, letter case.
It covers the primary menu in the header bar, its dropdowns, and its stacked copy in the mobile drawer. One control for all three — they are the same menu.
Face
The empty choice here reads Same as headings, and that is the default: the menu quietly follows whatever the heading face is. Change the headings and the menu comes with them.
Pick a face explicitly to break that link — a serif heading face with a sans-serif menu is a common and good-looking split, because a menu is an interface and an interface reads better in a sans.
Size
Default 1em — the body size. It is relative, so it moves with the body font size rather than drifting away from it.
| 0.85em – 0.9em | a quieter menu; useful when you have six or seven items |
| 1em | the default |
| 1.05em – 1.2em | a masthead menu, usually with the Nav below layout |
This is also the lever for making the header fit. The bar is as tall as its tallest item and as wide as its contents; dropping the menu to 0.9em buys you room without touching the padding.
Weight
Empty inherits the body’s weight — the stylesheet sets inherit here rather than 400, on purpose, so a menu in a light-weight body font does not suddenly snap to regular.
500 or 600 gives the menu a little more presence against a busy masthead. Watch the weight-must-exist rule if you are using an installed family.
Letter case and spacing
The small-caps menu — Uppercase with 0.06em to 0.1em of letter spacing, usually at 0.85em — is the standard newspaper navigation, and it is worth trying on the Nav below layout.
Uppercase without extra tracking looks cramped. Capitals are drawn assuming more room than lower case gets.
Not the footer menu
The footer menu is not covered here; it takes the body font at a small size. Adjust it with Additional CSS if you need to.