Fix #1203 icon alignment variable

src/definitions/elements/header.less

@@ -70,6 +70,9 @@
   display: inline-block;
   vertical-align: @contentAlignment;
 }
+.ui.header .icon + .content {
+  vertical-align: @contentIconAlignment;
+}
 
 /* Positioning */
 .ui.header:first-child {

src/themes/default/elements/header.variables

@@ -53,6 +53,7 @@
 
 /* Content */
 @contentAlignment: top;
+@contentIconAlignment: middle;
 
 /* Paragraph after Header */
 @nextParagraphTopMargin: 0em;