Create CSS styles for the given attributes
style_string(attr_names, state, geom)
| attr_names | character vector of e.g 'fill', 'stroke', 'font' |
|---|---|
| state | list including 'gc' (graphics context) |
| geom | which geometry has asked for a style |
style string ready for inclusion in an SVG tag e.g. "stroke: #ffffff; fill: #000000;"