Hi @authorjenna, give this a try.
#masthead {
display: flex;
flex-direction: column;
box-orient: vertical;
display: -webkit-box;
-webkit-flex-direction: column;
-webkit-box-orient: vertical;
display: -webkit-flex;
display: -moz-box;
-moz-flex-direction: column;
-moz-box-orient: vertical;
display: -ms-flexbox;
-ms-flex-direction: column;
-ms-box-orient: vertical;
}
#masthead .custom-header {
-webkit-box-ordinal-group: 1;
-moz-box-ordinal-group: 1;
-ms-flex-order: 1;
-webkit-order: 1;
order: 1;
}
#masthead .main-navigation {
-webkit-box-ordinal-group: 2;
-moz-box-ordinal-group: 2;
-ms-flex-order: 2;
-webkit-order: 2;
order: 2;
}
#masthead .site-branding {
-webkit-box-ordinal-group: 3;
-moz-box-ordinal-group: 3;
-ms-flex-order: 3;
-webkit-order: 3;
order: 3;
}
Just to let you know, although this advanced CSS3 is supported by the majority of modern browsers, in the case of Internet Explorer, it is only supported on version 10 and later, so anyone on 9 or earlier will not see this change.