[PATCH RESEND v2] css: Support for dark mode

[Jason A. Donenfeld]

It looks like the issue is that the string class for the light theme is:

.highlight .s {
    color: #dd2200;
    background-color: #fff0f0;
}

But for the dark theme it's:

@media (prefers-color-scheme: dark)
.highlight .s {
    color: #e6db74;
}

And so it winds up using the dark color, but with the light
background, because the dark theme doesn't specify a background.