Upload the main files

main.js

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+console.log("The extension is up and running");
+
+var images = document.getElementsByTagName('img')
+
+for (elt of images){
+   elt.src = `${browser.runtime.getURL("midnight.gif")}`
+   elt.alt = 'an alt text'
+}

manifest.json

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+{
+    "manifest_version":2,
+    "version":"0.0.1",
+    "name":"Midnightify",
+    "icons": {
+        "48": "midnight-icon.gif",
+        "96": "midnight-icon.gif"
+      },      
+    "content_scripts":[
+        {
+         "matches":["<all_urls>"],
+         "js":["main.js"]
+        }
+      ],
+      "web_accessible_resources": [
+            "midnight.gif"
+      ]
+  }
+