Change background colors of comment panels

Rolling forward change 250173 without the color swapping mistake.

This change is to address complaints that it is hard to see where the
code is and where the comment is at a quick glance on the diff view.
The proposition is to use a darker shade of grey for resolved comments
to provide better contrast against light and grey background color in
the diff view and a better glanceability of the resolved comments.

Change-Id: Id4015cfef9806f1e0e5ecf8458c2cde15b050ac7

polygerrit-ui/app/styles/themes/app-theme.html

diff --git a/polygerrit-ui/app/styles/themes/app-theme.html b/polygerrit-ui/app/styles/themes/app-theme.html
index 369af2b..0dec63f 100644
--- a/polygerrit-ui/app/styles/themes/app-theme.html
+++ b/polygerrit-ui/app/styles/themes/app-theme.html
@@ -59,15 +59,17 @@
   --view-background-color: var(--background-color-primary);
   /* unique background colors */
   --assignee-highlight-color: #fcfad6;
-  --comment-background-color: #fcfad6;
-  --robot-comment-background-color: #e8f0fe;
   --edit-mode-background-color: #ebf5fb;
   --emphasis-color: #fff9c4;
   --hover-background-color: rgba(161, 194, 250, 0.2);
   --primary-button-background-color: #2a66d9;
   --selection-background-color: rgba(161, 194, 250, 0.1);
   --tooltip-background-color: #333;
-  --unresolved-comment-background-color: #fcfaa6;
+  /* comment background colors */
+  --comment-background-color: #fef7f0;
+  --robot-comment-background-color: #e8f0fe;
+  --unresolved-comment-background-color: #e8eaed;
+  /* vote background colors */
   --vote-color-approved: #9fcc6b;
   --vote-color-disliked: #f7c4cb;
   --vote-color-neutral: #ebf5fb;

polygerrit-ui/app/styles/themes/dark-theme.html

diff --git a/polygerrit-ui/app/styles/themes/dark-theme.html b/polygerrit-ui/app/styles/themes/dark-theme.html
index 80243dc..1a49f18 100644
--- a/polygerrit-ui/app/styles/themes/dark-theme.html
+++ b/polygerrit-ui/app/styles/themes/dark-theme.html
@@ -49,15 +49,17 @@
       /*   empty, because inheriting from app-theme is just fine
       /* unique background colors */
       --assignee-highlight-color: #3a361c;
-      --comment-background-color: #0b162b;
-      --robot-comment-background-color: rgba(232, 234, 237, 0.08);
       --edit-mode-background-color: #5c0a36;
       --emphasis-color: #383f4a;
       --hover-background-color: rgba(161, 194, 250, 0.2);
       --primary-button-background-color: var(--link-color);
       --selection-background-color: rgba(161, 194, 250, 0.1);
       --tooltip-background-color: #111;
-      --unresolved-comment-background-color: #385a9a;
+      /* comment background colors */
+      --comment-background-color: #303134;
+      --robot-comment-background-color: #4B5058;
+      --unresolved-comment-background-color: #545146;
+      /* vote background colors */
       --vote-color-approved: #7fb66b;
       --vote-color-disliked: #bf6874;
       --vote-color-neutral: #597280;