WebUI Dark Mode: fix up empty list message in light/dark

Screenshots: https://imgur.com/a/EwqTzly

R=hcarmona@chromium.org
BUG=936524

Change-Id: I5ed3a43dc4273c1179eddca56a8138cf394c10a6
Reviewed-on: https://chromium-review.googlesource.com/c/1496246Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636676}

chrome/browser/resources/md_extensions/item_list.html

@@ -2,15 +2,15 @@
 
 <link rel="import" href="chrome://resources/cr_components/managed_footnote/managed_footnote.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_container_shadow_behavior.html">
-<link rel="import" href="chrome://resources/cr_elements/shared_style_css.html">
 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-a11y-announcer/iron-a11y-announcer.html">
 <link rel="import" href="item.html">
+<link rel="import" href="shared_style.html">
 
 <dom-module id="extensions-item-list">
   <template>
-    <style include="cr-shared-style">
+    <style include="shared-style">
       .items-container,
       #content-wrapper {
         --extensions-card-width: 400px;
@@ -35,6 +35,10 @@
         text-align: center;
       }
 
+      :host-context([dark]) .empty-list-message {
+        color: var(--cr-secondary-text-color);
+      }
+
       .items-container {
         --grid-gutter: 12px;
         --max-columns: 3;