Set sticky model column background to page base color

Uses --bg to prevent scrolling content from showing through while
keeping a consistent look with the page background.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

style.css

@@ -390,6 +390,7 @@ tbody tr.row-group-break td {
     position: sticky;
     left: 0;
     z-index: 1;
+    background: var(--bg);
 }
 .legend-section {
     margin: 2rem 0;