fix header invalid css property and update button styles

ui/scss/_vars.scss

@@ -73,6 +73,7 @@ $text-color: #000;
   --button-padding: $spacing-vertical * 2/3;
   --button-height: $spacing-vertical * 1.5;
   --button-intra-margin: $spacing-vertical;
+  --button-radius: 3px;
 
   /* Header */
   --header-bg: var(--color-bg);

ui/scss/component/_button.scss

@@ -20,7 +20,7 @@ $button-focus-shift: 12%;
   text-decoration: none;
   border: 0 none;
   text-align: center;
-  border-radius: 2px;
+  border-radius: var(--button-radius);
   text-transform: uppercase;
   .icon
   {

ui/scss/component/_header.scss

@@ -4,6 +4,8 @@
   color: var(--header-color);
   background: var(--header-bg);
   display: flex;
+  align-items: center;
+  justify-content: space-around;
   position: fixed;
   box-shadow: var(--box-shadow-layer);
   top: 0;
@@ -14,12 +16,11 @@
   box-sizing: border-box;
 }
 .header__item {
-  flex: 0 0 content;
   padding-left: $spacing-vertical / 4;
   padding-right: $spacing-vertical / 4;
   .button-alt {
       background: var(--header-button-bg) !important;
-      font-size: 18px;
+      font-size: 17px;
   }
   .button-alt:hover {
       background: var(--header-button-hover-bg) !important;