These options are located in the Theme Options > Header > Desktop Header tab. Then go to the Desktop Header Main Menu section.
Desktop Header Main Menu options control the styles for the main menu of the desktop header on your site.
The main menu setting of the desktop header is: See Setting Up The Menu
First of all, let’s find out about the terms used in this document.
MagicLine Height (Fig. 2)
You can set MagicLine Height if you select MagicLine: Top (Left in The Sidebar header) or MagicLine: Bottom (Right in The Sidebar header) of Main Menu Highlight Style.

MagicLine Height – Illustrated as 3. Controls the magicline height. In the sidebar header, controls the magicline width.
Highlight Item Background Solid Color (Fig. 3)

Main Menu Highlight Item Background Color – Illustrated as 4. Controls the menu item background color for the main menu highlight style.
Main Menu Highlight Item Text Color – Illustrated as 5. Controls the menu item text color for the main menu highlight style.
Highlight Item Background Gradient Color (Fig. 4)

Main Menu Highlight Item Background Gradient Colors – Illustrated as 6. Controls the menu item background gradient colors for the main menu highlight style.
Main Menu Highlight Item Background Gradient Angle – Illustrated as 7. Controls the menu item background gradient angle for the main menu highlight style. Degree(deg) unit.
Main Menu Dropdown
Main Menu Dropdown is a menu list that appears when you hover over the top-level item in the main menu. (Fig. 5)

Main Menu Dropdown Width On The 1 Column(Default) Sub-Menus Style
Sonnet offers a column of 4 customizable Dropdown to suit your site’s needs. See Menu Settings
Here, “Dropdown Multi-Column Sub-Menus” option in the top-level entry of the main menu means “Dropdown Width” when selected as “1 Column (Default)”. (Fig. 6)

Main Menu Dropdown Divider
Main Menu Dropdown Divider is divider line on dropdown menu items. (Fig. 7)

Dropdown Multi-Column Sub-Menus Column Divider
Sonnet offers a column of 4 customizable Dropdown to suit your site’s needs. See Menu Settings
“Dropdown Multi-Column Sub-Menus” in the top-level item of the main menu on the Appearance > Menus page means a divider between columns when selected as “2 columns” or “3 columns” or “Full Width (4 columns)”. (Fig. 6) (Fig. 8)

Sonnet offers a range of 4 customizable header layout versions (Fig. 10) to suit your site’s needs, and the Desktop Header Background options are suggested that must be controlled for that desktop header when setting the version of the desktop header set in the Desktop Header Layout.

Desktop header version 1
If you select the Desktop Header Layout option as Desktop header version 1, the Desktop Header Main Menu options you need to control are (Fig. 11):

Desktop Header Main Menu Item Alignment – Illustrated as A. Controls the Main Menu Item Alignment for the Desktop header version 1.
Desktop Header Main Menu Item – Illustrated as B. Controls the Main Menu Item for the Desktop header version 1.
Desktop Header Main Menu Dropdown – Illustrated as C. Controls the dropdown of Main Menu Item for the Desktop header version 1.
Desktop header version 2
If you select the Desktop Header Layout option as Desktop header version 2, the Desktop Header Main Menu options you need to control are (Fig. 12):

Desktop Header Main Menu Item Alignment – Illustrated as D. Controls the Main Menu Item Alignment for the Desktop header version 2.
Desktop Header Main Menu Item – Illustrated as E. Controls the Main Menu Item for the Desktop header version 2.
Desktop Header Main Menu Dropdown – Illustrated as F. Controls the dropdown of Main Menu Item for the Desktop header version 2.
Desktop header version 3
If you select the Desktop Header Layout option as Desktop header version 3, the Desktop Header Main Menu options you need to control are (Fig. 13):

Desktop Header Main Menu Item Alignment – Illustrated as G. Controls the Main Menu Item Alignment for the Desktop header version 3.
Main Menu Toggle Icon Container Height – Illustrated as H. Controls the main menu toggle icon item container height. (Fig. 14)
Desktop Header Main Menu Dropdown – Illustrated as I. Controls the Main Menu Item for the Desktop header version 3.
Desktop header version 4 (Sidebar Header)
If you select the Desktop Header Layout option as Desktop header version 4 (Sidebar Header), the Desktop Header Main Menu options you need to control are (Fig. 17):

Desktop Header Main Menu Item – Illustrated as L. Controls the Main Menu Item for the Sidebar Header.
Desktop Header Main Menu Dropdown – Illustrated as M. Controls the dropdown of Main Menu Item for the Sidebar Header.