use 100% 100% instead of contain for the background size

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

@@ -913,7 +913,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 			element.style.setProperty("background-position", "center");
 			element.style.setProperty("background-color", "transparent", "important");
 			element.style.setProperty("background-image", url, "important");
-			element.style.setProperty("background-size", "contain", "important");
+			element.style.setProperty("background-size", "100% 100%", "important");
 			element.style.setProperty("background-origin", "content-box", "important");
 			element.style.setProperty("background-repeat", "no-repeat", "important");
 		}