This should do it, i added the color change as well:
@media (max-width: 768px) {
#masthead {
display: block !important;
opacity: 1;
}
.inside-header .site-logo {
display: none;
}
.inside-header.grid-container {
padding-bottom: 0;
}
.header-widget {
margin-bottom: 0 !important;
font-size: 0;
}
.main-navigation.mobile-header-navigation {
background-color: #fff;
}
}