diff --git a/regress/theme-report.sh b/regress/theme-report.sh
new file mode 100755
index 000000000..4c42244b4
--- /dev/null
+++ b/regress/theme-report.sh
@@ -0,0 +1,57 @@
+#!/bin/sh
+
+# Test that the theme reported to a pane (mode 2031 / DSR 996) follows the
+# terminal's reported theme, not a guess from the background colour.
+#
+# An inner client is attached inside an outer tmux pane. The outer pane has a
+# light background, so the outer server answers the inner client's DSR 996 with
+# light. The inner server is given a dark window background, so a guess from the
+# background colour would say dark. A pane in the inner server then queries DSR
+# 996: the answer must be light (2), following the client's reported theme, not
+# dark (1) from the background.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+INNER="$TEST_TMUX -LtestI$$ -f/dev/null"
+OUTER="$TEST_TMUX -LtestO$$ -f/dev/null"
+$INNER kill-server 2>/dev/null
+$OUTER kill-server 2>/dev/null
+
+TMP=$(mktemp)
+trap "rm -f $TMP; $INNER kill-server 2>/dev/null; $OUTER kill-server 2>/dev/null" \
+	0 1 15
+
+# Inner server, dark background, keep panes alive across the query respawn.
+$INNER new-session -d -x80 -y24 || exit 1
+$INNER set -g remain-on-exit on
+$INNER set -g window-style 'bg=black'
+
+# Outer server with a light background, running the inner client attached so the
+# inner client has a real terminal that reports a theme.
+$OUTER new-session -d -x80 -y24 || exit 1
+$OUTER set -g window-style 'bg=white'
+$OUTER new-window "$INNER attach" || exit 1
+sleep 2
+
+# Query DSR 996 from an inner pane and capture the CSI ? 997 ; Ps n reply.
+$INNER respawnw -k -t:0 -- sh -c "
+	exec 2>/dev/null
+	stty raw -echo
+	printf '\033[?996n'
+	dd bs=1 count=9 2>/dev/null | cat -v > $TMP
+	sleep 1
+"
+sleep 2
+
+actual=$(cat "$TMP")
+expected='^[[?997;2n'   # 2 = light (from the terminal), not 1 = dark (from bg)
+
+if [ "$actual" = "$expected" ]; then
+	[ -n "$VERBOSE" ] && echo "[PASS] reported terminal theme ($actual)"
+	exit 0
+fi
+
+echo "[FAIL] expected '$expected' (light, from terminal), got '$actual'"
+exit 1