refactor(frontend): getBgColorを共通化 (#14782)

* refactor: getBgColor関数の切り出し + fix types (taiyme#291)

* move thing

* revert unnecesary changes

---------

Co-authored-by: taiy <53635909+taiyme@users.noreply.github.com>

packages/frontend/src/scripts/get-bg-color.ts

@@ -0,0 +1,18 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import tinycolor from 'tinycolor2';
+
+export const getBgColor = (elem?: Element | null | undefined): string | null => {
+	if (elem == null) return null;
+
+	const { backgroundColor: bg } = window.getComputedStyle(elem);
+
+	if (bg && tinycolor(bg).getAlpha() !== 0) {
+		return bg;
+	}
+
+	return getBgColor(elem.parentElement);
+};