WebUI Dark Mode: update paper-tabs selection and label colors

R=dpapad@chromium.org
BUG=883049

Change-Id: Ia9977c7eada06142181b2f4bc331c269ba88aeef
Reviewed-on: https://chromium-review.googlesource.com/c/1429664
Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625330}

ui/webui/resources/cr_elements/paper_tabs_style_css.html

@@ -11,6 +11,11 @@
         --paper-tabs-selection-bar-color: var(--google-blue-600);
       }
 
+      :host-context([dark]) paper-tabs {
+        /* TODO(dbeam): verify these colors with Namrata. */
+        --paper-tabs-selection-bar-color: var(--google-blue-refresh-300);
+      }
+
       paper-tab {
         --paper-tab-content: {
           color: var(--google-blue-600);
@@ -19,6 +24,12 @@
           color: var(--cr-secondary-text-color);
         };
       }
+
+      :host-context([dark]) paper-tab {
+        --paper-tab-content: {
+          color: var(--google-blue-refresh-300);
+        };
+      }
     </style>
   </template>
 </dom-module>