[MD] Change appearance of incognito NTP to MD spec.

chrome/browser/resources/ntp4/new_incognito_tab_theme.css

Index: chrome/browser/resources/ntp4/new_incognito_tab_theme.css
diff --git a/chrome/browser/resources/ntp4/new_incognito_tab_theme.css b/chrome/browser/resources/ntp4/new_incognito_tab_theme.css
index dd31189d50320ef96ef4106e72d6da01d13b20c5..175f435d0ab99278e5eb8cb5ffa0a353c4fe5f41 100644
--- a/chrome/browser/resources/ntp4/new_incognito_tab_theme.css
+++ b/chrome/browser/resources/ntp4/new_incognito_tab_theme.css
@@ -6,13 +6,21 @@
 html {
   background-attachment: fixed;
   background-color: ${colorBackground};
-  background-image: url(chrome://theme/IDR_THEME_NTP_BACKGROUND?${themeId});
   background-position: ${backgroundBarDetached};
   background-repeat: ${backgroundTiling};
   height: 100%;
   overflow: auto;
 }
 
+html${hasCustomBackground} {
+  background-image: url(chrome://theme/IDR_THEME_NTP_BACKGROUND?${themeId});
+}
+
+html${hasCustomBackground} .content {
+  border-radius: 2px;
+  box-shadow: 0 4px 6px 1px rgba(0, 0, 0, 0.4);
+}
+
 #attribution-img {
   content: url(chrome://theme/IDR_THEME_NTP_ATTRIBUTION?${themeId});
 }