Change the theme header to breadcrumbs.

client/my-sites/theme/main.jsx

@@ -40,9 +40,9 @@ import QuerySitePlans from 'calypso/components/data/query-site-plans';
 import QuerySitePurchases from 'calypso/components/data/query-site-purchases';
 import QueryUserPurchases from 'calypso/components/data/query-user-purchases';
 import SyncActiveTheme from 'calypso/components/data/sync-active-theme';
-import HeaderCake from 'calypso/components/header-cake';
 import InlineSupportLink from 'calypso/components/inline-support-link';
 import Main from 'calypso/components/main';
+import NavigationHeader from 'calypso/components/navigation-header';
 import PremiumGlobalStylesUpgradeModal from 'calypso/components/premium-global-styles-upgrade-modal';
 import ThemeSiteSelectorModal from 'calypso/components/theme-site-selector-modal';
 import { THEME_TIERS } from 'calypso/components/theme-tier/constants';
@@ -1540,6 +1540,11 @@ class ThemeSheet extends Component {
 			'is-removed': isRemoved,
 		} );
 
+		const navigationItems = [
+			{ label: 'Themes', href: siteSlug ? `/themes/${ siteSlug }` : '/themes' },
+			{ label: title },
+		];
+
 		return (
 			<Main className={ className }>
 				<QueryCanonicalTheme themeId={ this.props.themeId } siteId={ siteId } />
@@ -1589,14 +1594,7 @@ class ThemeSheet extends Component {
 				/>
 				<ThanksModal source="details" themeId={ this.props.themeId } />
 				<ActivationModal source="details" />
-				<div className="theme__sheet-action-bar-container">
-					<HeaderCake
-						className="theme__sheet-action-bar"
-						backText={ translate( 'Back to themes' ) }
-						onClick={ this.goBack }
-						alwaysShowBackText
-					/>
-				</div>
+				<NavigationHeader navigationItems={ navigationItems } />
 				<div className={ columnsClassName }>
 					<div className="theme__sheet-column-header">
 						{ pageUpsellBanner }

client/my-sites/theme/style.scss

@@ -61,6 +61,20 @@ $button-border: 4px;
 			overflow-y: auto;
 		}
 	}
+
+	.theme__sheet .navigation-header {
+		margin: 0 auto;
+		max-width: $theme-sheet-content-max-width;
+		padding: 24px 44px;
+
+		@media screen and (min-width: 960px) {
+			padding: 24px 20px;
+		}
+
+		@media screen and (min-width: 1806px) {
+			padding: 24px 0;
+		}
+	}
 }
 
 .main.theme__sheet {
@@ -280,10 +294,6 @@ $button-border: 4px;
 	grid-area: preview;
 }
 
-.theme__sheet-action-bar-container {
-	box-shadow: 0 1px var(--color-border-subtle);
-}
-
 .header-cake.card.theme__sheet-action-bar {
 	box-shadow: none;
 	box-sizing: content-box;