Try this as your full CSS:
.header-widget {
float: none;
width: 600px;
max-width: 100%;
margin-left: auto;
margin-right: auto;
display: inline-block;
}
.header-widget .widget .search-field {
width: 70%;
}
.header-widget .widget_search .search-submit {
display: inline-block;
}
@media (min-width: 769px) {
.site-logo {
float: left;
margin-right: 20px;
}
}