prevent overwiting existing background color

lib/single-file/single-file-core.js

@@ -907,7 +907,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 			element.style.setProperty("background-blend-mode", "normal", "important");
 			element.style.setProperty("background-clip", "content-box", "important");
 			element.style.setProperty("background-position", style && style["background-position"] ? style["background-position"] : "center", "important");
-			element.style.setProperty("background-color", "transparent", "important");
+			element.style.setProperty("background-color", style && style["background-color"] ? style["background-color"] : "transparent", "important");
 			element.style.setProperty("background-image", url, "important");
 			element.style.setProperty("background-size", style && style["background-size"] ? style["background-size"] : "100% 100%", "important");
 			element.style.setProperty("background-origin", "content-box", "important");