Pin Category and Domain cells to consistent background

Zebra striping was leaking into rowspan cells when a domain group
happened to start on an even row (visible for Common sense).

app.py

@@ -727,12 +727,14 @@ body { background: var(--bg) !important; }
     letter-spacing: 0.06em;
     vertical-align: middle;
     min-width: 8rem;
+    background: var(--surface) !important;
 }
 .evaluation-suite-table .domain-cell {
     color: var(--text-dim);
     font-size: 0.82rem;
     vertical-align: middle;
     overflow-wrap: anywhere;
+    background: var(--surface) !important;
 }
 .evaluation-suite-table .benchmark-cell { width: 12.5rem; overflow-wrap: anywhere; }
 .evaluation-suite-table .description-cell { min-width: 0; overflow-wrap: anywhere; }