dark_theme_css: Fix border color for tables in message/chat.

Add a css rule in dark_theme.css for tables inside message/chat.
This rule sets a brighter color for these type of tables so that
it's more visible when the user is using dark theme.

Fixes zulip#29856

web/styles/dark_theme.css

@@ -799,6 +799,14 @@
         border-color: hsl(0deg 0% 0% / 20%);
     }
 
+    /* This sets a brighter color for tables inside
+       messages when using dark mode. */
+    .message_content.rendered_markdown table,
+    .message_content.rendered_markdown table th,
+    .message_content.rendered_markdown table td {
+        border-color: hsl(0deg 0% 100% / 80%);
+    }
+
     .overlay-message-row
         .message_header_private_message
         .message_label_clickable {