problem overriding background colour

Hi - I'm trying to override the background colour of a button with class .button. However, the code I'm using in my child theme is being overridden by the following:

button, html input[type="button"], input[type="reset"], input[type="submit"], .button, .button:visited {
    color: #ffffff;
    background-color: #666666;
}

According to Chrome dev tools, this is not coming from a style sheet, but from this link http://localhost:8888/northumberlandgreen/thank-you-for-subscribing/?preview_id=107&preview_nonce=50ae75d03b&_thumbnail_id=-1&preview=true

I've never seen this before, and I can't work out why the code is being applied over the code in my child theme. I can override it using !important, but that then overrides the background cover for the hover state. I could probably add an ID and use that to solve the problem, but I'd really like to know where this code is coming from. Can you help?

Thank you!
Caroline