Unify top panel header styling

public/index.html

@@ -17,7 +17,7 @@
       <button class="sidebar-overlay" id="sidebarOverlay" type="button" aria-label="Close sidebar"></button>
 
       <aside class="control-panel sidebar-drawer" id="sidebarDrawer" aria-label="Catalog controls">
-        <div class="sidebar-drawer-header">
+        <div class="panel-header sidebar-drawer-header">
           <span class="sidebar-drawer-title">Catalog controls</span>
           <button class="sidebar-close-button panel-control-button" id="sidebarCloseButton" type="button" aria-label="Close sidebar" title="Close sidebar">
             <svg class="sidebar-icon" aria-hidden="true"><use href="/icons.svg#close"></use></svg>
@@ -97,7 +97,7 @@
       </aside>
 
       <section class="product-panel secondary-panel" aria-label="Product details">
-        <div class="column-header">
+        <div class="panel-header column-header">
           <span>Product details</span>
           <button class="column-toggle" id="secondaryPanelToggle" type="button" aria-expanded="true" aria-controls="fieldGrid" aria-label="Collapse product details" title="Collapse product details">‹</button>
         </div>
@@ -105,7 +105,7 @@
       </section>
 
       <section class="data-panel main-workspace" aria-label="Catalog workspace">
-        <div class="column-header workspace-header">
+        <div class="panel-header column-header workspace-header">
           <span>Main workspace</span>
           <button class="column-toggle" id="mainWorkspaceToggle" type="button" aria-expanded="true" aria-controls="variantGrid" aria-label="Collapse main workspace" title="Collapse main workspace">›</button>
         </div>