UI: Update colors theme for tasks

- Use colors matching the theme of labels in change-list view
- Use red color for failed tasks instead of orange
- Make colors compatible with dark theme

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

Change-Id: I5d033eb528d8829bbfa86112724db6d91435b8d1

gr-task-plugin/gr-task-plugin-tasks_html.js

diff --git a/gr-task-plugin/gr-task-plugin-tasks_html.js b/gr-task-plugin/gr-task-plugin-tasks_html.js
index dd5755b..60fbd32 100644
--- a/gr-task-plugin/gr-task-plugin-tasks_html.js
+++ b/gr-task-plugin/gr-task-plugin-tasks_html.js
@@ -20,12 +20,15 @@
   <template is="dom-if" if="[[_can_show(show_all, task)]]">
     <li>
       <style>
-        /* Matching colors with core code. */
+        /* Matching colors with core theme. */
         .green {
-          color: #9fcc6b;
+          color: var(--success-foreground);
         }
         .red {
-          color: #FFA62F;
+          color: var(--error-foreground);
+        }
+        .orange {
+          color: var(--warning-foreground);
         }
         .links {
           color: var(--link-color);

gr-task-plugin/gr-task-plugin.js

diff --git a/gr-task-plugin/gr-task-plugin.js b/gr-task-plugin/gr-task-plugin.js
index 671aa71..2ac355d 100644
--- a/gr-task-plugin/gr-task-plugin.js
+++ b/gr-task-plugin/gr-task-plugin.js
@@ -181,7 +181,7 @@
         break;
       case 'WAITING':
         icon.id = 'gr-icons:pause';
-        icon.color = 'red';
+        icon.color = 'orange';
         icon.tooltip = 'Waiting';
         break;
       case 'DUPLICATE':