I can’t remove the grey border of Header

Hi, @yaowinnie

Try adding this in Dashboard – Appereance – Customize – Additional CSS in the Customizer

.ast-primary-header-bar, .ast-above-header-bar  {
    border-bottom: none;
}

You can learn how to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.

Hope this helps,
Kind regards!