How to increase the width of submenu items in default header?

You can adjust the width of the submenu items using custom CSS.

Add the below CSS code to your WordPress theme by following these steps:

1. Navigate to Appearance > Customize in your WordPress Dashboard.
2. Select Additional CSS.
3. Paste the CSS code into the provided text area.
4. Click Publish to save your changes.

#top-menu .sub-menu {
    width: 300px; 
}

#top-menu .sub-menu li{
    width: 100%;
}

#top-menu .sub-menu li a {
    width: 100%;
}