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;
}