diff --git a/src/service-worker.js b/src/service-worker.js
index d99356a..4a9be66 100644
--- a/src/service-worker.js
+++ b/src/service-worker.js
@@ -10,23 +10,30 @@ let injectedCSS = {};
  * @param {string} text
  */
 async function insertOrReplaceCss(tabId, category, text) {
-	if (tabId in injectedCSS && category in injectedCSS[tabId]) {
-		if (injectedCSS[tabId][category] == text) {
-			return;
-		}
+	const previousCss = injectedCSS[tabId]?.[category];
+	if (previousCss == text) {
+		return;
+	}
+
+	// Insert new CSS...
+	await chrome.scripting.insertCSS({
+		target: { tabId },
+		css: text
+	});
+
+	if (previousCss) {
+		// ...and remove old CSS *afterwards* to prevent FOUT
 		await chrome.scripting.removeCSS({
 			target: { tabId },
-			css: injectedCSS[tabId][category]
+			css: previousCss
 		});
 	}
+
 	if (!(tabId in injectedCSS)) {
 		injectedCSS[tabId] = {};
 	}
+
 	injectedCSS[tabId][category] = text;
-	await chrome.scripting.insertCSS({
-		target: { tabId },
-		css: text
-	});
 }
 
 // Do the thing! (Generate stylesheet and inject into active tab)