[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..496b95b99f8ef3b5fe870d93e3f829ff71ebf948 100644
--- a/chrome/browser/resources/ntp4/new_incognito_tab_theme.css
+++ b/chrome/browser/resources/ntp4/new_incognito_tab_theme.css
@@ -6,17 +6,20 @@
 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;
 }
 
-#attribution-img {
-  content: url(chrome://theme/IDR_THEME_NTP_ATTRIBUTION?${themeId});
+html[hascustombackground='true'] {
+  background-image: url(chrome://theme/IDR_THEME_NTP_BACKGROUND?${themeId});
 }
 
 html[bookmarkbarattached='true'] {
   background-position: ${backgroundBarAttached};
 }
+
+#attribution-img {
+  content: url(chrome://theme/IDR_THEME_NTP_ATTRIBUTION?${themeId});
+}