Make icons react to color-scheme properly.

Note that due to https://bugzilla.mozilla.org/show_bug.cgi?id=1779457 /
w3c/csswg-drafts#7213, prefers-color-scheme
works fine even if the user has an explicitly light theme or so.

This fixes mozilla#2583 too.

src/js/content-script.js

@@ -24,11 +24,12 @@ async function addMessage(message) {
   divElement.innerText = message.text;
 
   const imageElement = document.createElement("img");
-  const imagePath = browser.runtime.getURL("/img/container-site-d-24.png");
+  const imagePath = browser.runtime.getURL("/img/multiaccountcontainer-16.svg");
   const response = await fetch(imagePath);
   const blob = await response.blob();
   const objectUrl = URL.createObjectURL(blob);
   imageElement.src = objectUrl;
+  imageElement.width = imageElement.height = 24;
   divElement.prepend(imageElement);
 
   document.body.appendChild(divElement);

src/manifest.json

@@ -5,8 +5,8 @@
   "incognito": "not_allowed",
   "description": "__MSG_extensionDescription__",
   "icons": {
-    "48": "img/container-site-d-48.png",
-    "96": "img/container-site-d-96.png"
+    "48": "img/multiaccountcontainer-16.svg",
+    "96": "img/multiaccountcontainer-16.svg"
   },
   "homepage_url": "https://github.com/mozilla/multi-account-containers#readme",
   "permissions": [
@@ -118,7 +118,7 @@
     "default_popup": "popup.html",
     "theme_icons": [
       {
-        "light": "img/multiaccountcontainer-16-dark.svg",
+        "light": "img/multiaccountcontainer-16.svg",
         "dark": "img/multiaccountcontainer-16.svg",
         "size": 32
       }
@@ -151,7 +151,7 @@
   ],
   "default_locale": "en",
   "web_accessible_resources": [
-    "/img/container-site-d-24.png"
+    "/img/multiaccountcontainer-16.svg"
   ],
   "options_ui": {
     "page": "options.html",