MD History: Address design nits

chrome/browser/resources/md_history/synced_device_manager.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.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-item/paper-item.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-menu.html">
 <link rel="import" href="chrome://resources/html/cr/ui/position_util.html">
 <link rel="import" href="chrome://history/shared_style.html">
 <link rel="import" href="chrome://history/synced_device_card.html">
 
 <dom-module id="history-synced-device-manager">
@@ skipping 22 matching lines @@
       #sign-in-guide {
         align-items: center;
         display: flex;
         flex-direction: column;
         justify-content: center;
         overflow-x: hidden;
         text-align: center;
       }
 
       #sign-in-promo {
-        color: #333;
+        color: var(--primary-text-color);
         font-size: 28px;
         margin-top: 40px;
       }
 
       #sign-in-promo-desc {
         color: #848484;
         font-size: 16px;
         margin-top: 10px;
       }
 
@@ skipping 26 matching lines @@
       <div id="illustration"></div>
       <div id="sign-in-promo">$i18n{signInPromo}</div>
       <div id="sign-in-promo-desc">$i18n{signInPromoDesc}</div>
       <paper-button id="sign-in-button" on-tap="onSignInTap_">
         $i18n{signInButton}
       </paper-button>
     </div>
   </template>
   <script src="chrome://history/synced_device_manager.js"></script>
 </dom-module>