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
diff --git a/polygerrit-ui/app/elements/diff/gr-confirm-delete-comment-dialog/gr-confirm-delete-comment-dialog.html b/polygerrit-ui/app/elements/diff/gr-confirm-delete-comment-dialog/gr-confirm-delete-comment-dialog.html
index 1a6245d..22fd2aa 100644
--- a/polygerrit-ui/app/elements/diff/gr-confirm-delete-comment-dialog/gr-confirm-delete-comment-dialog.html
+++ b/polygerrit-ui/app/elements/diff/gr-confirm-delete-comment-dialog/gr-confirm-delete-comment-dialog.html
@@ -46,7 +46,7 @@
         width: 73ch; /* Add a char to account for the border. */
 
         --iron-autogrow-textarea {
-          border: 1px solid #cdcdcd;
+          border: 1px solid var(--border-color);
           box-sizing: border-box;
           font-family: var(--monospace-font-family);
         }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-preferences/gr-diff-preferences.html b/polygerrit-ui/app/elements/diff/gr-diff-preferences/gr-diff-preferences.html
index d4bcef4..68c4d39 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-preferences/gr-diff-preferences.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-preferences/gr-diff-preferences.html
@@ -45,7 +45,7 @@
         padding: 1em 1.5em;
       }
       .header {
-        border-bottom: 1px solid #ddd;
+        border-bottom: 1px solid var(--border-color);
         font-family: var(--font-family-bold);
       }
       .mainContainer {
@@ -65,7 +65,7 @@
         flex: 1;
       }
       .actions {
-        border-top: 1px solid #ddd;
+        border-top: 1px solid var(--border-color);
         display: flex;
         justify-content: flex-end;
       }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
index c7a1d86..79056f6 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html
@@ -53,12 +53,12 @@
       gr-diff {
         border: none;
         --diff-container-styles: {
-          border-bottom: 1px solid #eee;
+          border-bottom: 1px solid var(--border-color);
         }
       }
       gr-fixed-panel {
         background-color: #fff;
-        border-bottom: 1px #eee solid;
+        border-bottom: 1px solid var(--border-color);
         z-index: 1;
       }
       header,
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index 2aaaae5..9fd564c 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -58,7 +58,7 @@
       }
       table {
         border-collapse: collapse;
-        border-right: 1px solid #ddd;
+        border-right: 1px solid var(--border-color);
         table-layout: fixed;
       }
       .lineNum {