adjust main manu font size

Can the font size be adjusted in the main menu at the top of the home page?

You can override with this custom CSS:

.navbar-default .navbar-nav > li > a, .navbar-default .navbar-nav > li > span {
    font-size: 14px !important;
}

and change to your desired value.

excuse me，Where to past these code?

You can put into the file: /templates/ja_oslo/css/custom.css (Create this file if it doesn't exist)

Thank you so much.It's helpful!! great!!