Your browser has an even better feature where you can specify client-side CSS. I use that to override website HTML all the time. Here's an example:

* {
    color: white !important;
    background-color: black !important;
    FONT-FAMILY: sans-serif;
    FONT-SIZE: 13px;
    FONT-WEIGHT: normal;
}
a { text-decoration: underline; }
a:link { background-color: #000; }
a:visited { background-color: #131; text-decoration: strikethrough; }
form { display: inline; }
textarea {
    width: 100%;
    height: 25em;
}
pre { font-family: monospace; }
tt * {
    font-family: monospace;
    color: green;
}
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