WebUI Dark Mode: update passwords export dialog progress colors

Screenshot: https://imgur.com/a/REuT5VD

R=hcarmona@chromium.org
BUG=926658

Change-Id: I7ec299554e136a7fd7a948bbeadba73b0af829d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506479
Commit-Queue: Dan Beam (slow; perfing) <dbeam@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Auto-Submit: Dan Beam (slow; perfing) <dbeam@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638371}

chrome/browser/resources/settings/autofill_page/passwords_export_dialog.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 
 <link rel="import" href="chrome://resources/cr_elements/cr_dialog/cr_dialog.html">
+<link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/iron-flex-layout-classes.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-progress/paper-progress.html">
@@ -10,9 +11,17 @@
   <template>
     <style include="settings-shared iron-flex">
       paper-progress {
-        width: 100%;
         --paper-progress-active-color: var(--google-blue-500);
+        width: 100%;
       }
+
+      :host-context([dark]) paper-progress {
+        /* TODO(dbeam): this is the same as downloads (and probably anywhere
+         * else that uses paper-progress). Should we make something like a
+         * paper_progress_style_css.html? */
+        --paper-progress-active-color: var(--google-blue-refresh-300);
+      }
+
       .action-button {
         margin-inline-start: 8px;
       }