Use the same border color throughout the app

With this change, all CSS border declarations that specify some shade of
gray are modified to use the one unified --border-color declaration in
app-theme.html.

Further followup with UX is required to ensure that the shade of gray
chosen is the correct one -- for now, the most-used one is used.

Change-Id: I2134ebf4a459ba040e01856c11389780307d8648

polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html

diff --git a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
index ecd62cf..6597f4a 100644
--- a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
+++ b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.html
@@ -39,7 +39,7 @@
       }
       gr-fixed-panel {
         background-color: #ebf5fb;
-        border-bottom: 1px #ddd solid;
+        border-bottom: 1px var(--border-color) solid;
         z-index: 1;
       }
       header,
@@ -62,7 +62,7 @@
         }
       }
       .textareaWrapper {
-        border: 1px solid #ddd;
+        border: 1px solid var(--border-color);
         border-radius: 3px;
         margin: var(--default-horizontal-margin);
       }