WebUI Dark Mode: fix a contrast problem on extension error page

Also, make URLs wrap more gracefully.

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

R=rdevlin.cronin@chromium.org
BUG=937159

Change-Id: Ied0b93cc094c935e45cfad4745007db723e1267e
Reviewed-on: https://chromium-review.googlesource.com/c/1496596Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636699}

chrome/browser/resources/md_extensions/error_page.html

@@ -117,8 +117,12 @@
       .stack-trace-container li.selected,
       .stack-trace-container li:hover {
         background: var(--google-blue-100);
+        color: var(--google-grey-900);  /* Same in light & dark modes. */
       }
 
+      /* TODO(crbug.com/937159): all of the code shown in the errors page is
+       * themed for "light mode" and needs to be updated to co-exit with "dark
+       * mode" when specs are available. */
       extensions-code-section {
         background: white;
         height: 200px;
@@ -132,6 +136,10 @@
       .start:focus {
         outline: none;
       }
+
+      .context-url {
+        word-wrap: break-word;
+      }
     </style>
     <div class="page-container" id="container">
       <div class="page-content">