How to change the style of navigation menu in mobile

Hi there,

Give this CSS a shot:

#mobile-header .menu-toggle {
    border: 2px solid #000;
    border-radius: 20px;
    line-height: 40px !important;
}

Adding CSS: https://docs.generatepress.com/article/adding-css/

It worked but i am not able to adjust the button size even after changing line height, here is the screenshot

Try this instead:

#mobile-header .menu-toggle {
    border: 2px solid #000;
    border-radius: 20px;
    line-height: 35px;
    align-self: center;
}