Add background color with contrast for robot comments in dark theme

https://imgur.com/IZfKBPY

Change-Id: I99fd0076e0d4eebd44fbf443cbffb395e3cf5e1f

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 e50890f..cfc1f62 100644
--- a/polygerrit-ui/app/styles/themes/dark-theme.html
+++ b/polygerrit-ui/app/styles/themes/dark-theme.html
@@ -50,7 +50,7 @@
       /* unique background colors */
       --assignee-highlight-color: #3a361c;
       --comment-background-color: #0b162b;
-      --robot-comment-background-color: #e8f0fe;
+      --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);