user themes cannot be modified via JS

src/js/colorModeJS.js

@@ -12,28 +12,32 @@ window.setColors = function(colorMap) {
 	let props = ["color", "backgroundColor", "backgroundImage"];
 	let styleSheetArray = Array.from(document.styleSheets);
 	styleSheetArray.forEach(styleSheet => {
-		let cssRules = Array.from(styleSheet.cssRules);
-		cssRules.forEach(cssRule => {
-			if (cssRule.type === 1) {
-				props.forEach(propName => {
-					let currentValue = cssRule.style[propName];
-					if (
-						currentValue !== "" &&
-						currentValue !== "inherit" &&
-						currentValue !== "transparent") {
-						let newVal = colorMap[currentValue];
-						if (typeof newVal !== "undefined") {
-							cssRule.style[propName] = newVal;
-							originalState.push({
-								element: cssRule,
-								propName: propName,
-								value: currentValue
-							});
+		try {
+			let cssRules = Array.from(styleSheet.cssRules);
+			cssRules.forEach(cssRule => {
+				if (cssRule.type === 1) {
+					props.forEach(propName => {
+						let currentValue = cssRule.style[propName];
+						if (
+							currentValue !== "" &&
+							currentValue !== "inherit" &&
+							currentValue !== "transparent") {
+							let newVal = colorMap[currentValue];
+							if (typeof newVal !== "undefined") {
+								cssRule.style[propName] = newVal;
+								originalState.push({
+									element: cssRule,
+									propName: propName,
+									value: currentValue
+								});
+							}
 						}
-					}
-				});
-			}
-		});
+					});
+				}
+			});
+		} catch (error) {
+			// No need to do anthing, this is expected when there are user style sheets.
+		}
 	});
 	return originalState;
 };