The .css file from the popup.html file in the chrome extension is not connected, maybe you need to register it in manifest?

    1 answer 1

    From the Google Guide:

    If you need to use your content_scripts field, please use your content script.

    { "name": "My extension", ... "content_scripts": [{ "matches": ["http://www.google.com/*"], "css": ["mystyles.css"], "js": ["jquery.js", "myscript.js"] }], ... }

    • Yes, this is how css is connected for elements that are dynamically created when content_script is running, but popup is not injected into the page code - Igor
    • For Firefox extensions, simply connect the styles and script in the html-document itself (popup.html). - topvova