fix(app): treat a black instance theme colour as unset (#3502)

packages/instance_bootstrap/src/ThemeColor.ts

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+const HEX_THEME_COLOR_PATTERN = /^#?([0-9a-f]{3}|[0-9a-f]{6})$/i;
+const UNSET_THEME_COLOR = '#000000';
+
+export function normalizeThemeColor(value: unknown): string | null {
+	if (typeof value !== 'string') return null;
+	const match = HEX_THEME_COLOR_PATTERN.exec(value.trim());
+	if (match === null) return null;
+	const digits = match[1].toLowerCase();
+	const color = `#${digits.length === 3 ? digits.replace(/./g, '$&$&') : digits}`;
+	return color === UNSET_THEME_COLOR ? null : color;
+}