Set lineNum column background even when wrapping

https://imgur.com/a/So5b086
Change-Id: I58bda797e8d46c1767824a2165a6684e6c551a63
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_html.ts b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_html.ts
index 5b248da..98956c9 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_html.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff_html.ts
@@ -63,6 +63,7 @@
     table {
       border-collapse: collapse;
       table-layout: fixed;
+      background-color: var(--diff-blank-background-color);
     }
 
     /*