Fix Options UI background colour

Author: ExE-Boss

src/browser-style-supplement/extension-options.css

@@ -7,7 +7,6 @@
 html {
 	font:	message-box;
 	padding:	6px;
-	background: #FBFBFB;
 }
 
 .browser-style {

src/options/options.js

@@ -19,11 +19,19 @@
 
 document.addEventListener("DOMContentLoaded", async () => {
 	return Promise.all([
+		fixBackgroundColor(),
 		i18nInit(),
 		initOptions()
 	]);
 });
 
+async function fixBackgroundColor() {
+	let browserInfo = await browser.runtime.getBrowserInfo();
+	if (browserInfo.name === "Firefox" && browserInfo.version.localeCompare("57", {numeric:true}) < 0) {
+		document.documentElement.style.background = "#FBFBFB";
+	}
+}
+
 async function initOptions() {
 	let themes = ["default", "photon", "australis", "classic", "pastel-svg", "aero"];
 	let data = (await browser.storage.sync.get({