Allow to add an embeded style element with css definitions

[hansroland]

Today all browsers support light and dark modes. Therefore it should be possible to generate SVG files, that display nicely in both modes.

An example motivated by these SO answers is :

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
    "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
    <svg xmlns="http://www.w3.org/2000/svg" width="200" xmlns:xlink="http://www.w3.org/1999/xlink" height="200" version="1.11.1">
    <style>
        line { stroke: black; stroke-width: "1";}
        rect { fill: white; }
        @media (prefers-color-scheme: dark) {
            line { stroke: white; stroke-width: "1";}
            rect { fill: black; }
        }
    </style>
    <rect width="100%" height="100%" fill="currentColor"/>
    <line y1="199" x1="1" y2="199" x2="199"/>
    <line y1="199" x1="1" y2="1" x2="1"/>
    <line y1="199" x1="199" y2="1" x2="199"/>
    <line y1="1" x1="1" y2="1" x2="199"/>
    <line y1="139" x1="15" y2="177" x2="115"/>
    <line y1="147" x1="147" y2="177" x2="115"/>
                       ... more lines ...
    </svg>

I failed to construct such a SVG file with svg-builder. I can create a style element with the style_ function, however, I'm unable to create the css contents.