How to change header main menu font size? I tried #shopify-section-sections--18158375403629__header .list-menu__item { font-size: 20px; font-weight: 600; text-transform: uppercase; letter-spacing: 0.8px; } but didn't work

You already have this code in the theme, so just find and change font-size value

#shopify-section-sections--18158375403629__header .list-menu__item {
    font-size: 13px;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.8px;
}

Or you can add this code to Custom CSS

#shopify-section-sections--18158375403629__header .list-menu__item {
    font-size: 20px !important;
}

@Filpup … Simply follow..

Go to: Online Store → Themes → Edit Code → base.css or add to Custom CSS

/* Main nav menu font */
.header__inline-menu .list-menu--inline .list-menu__item--link {
  font-size: 20px !important;
  font-weight: 600 !important;
  text-transform: uppercase !important;
  letter-spacing: 0.8px !important;
}

Or, the simplest way — use the Theme Customizer:

  1. Online Store → Themes → Customize

  2. Click Header section

  3. Look for “Navigation font size” or “Menu” typography settings

  4. Change directly there — no code needed