Support currentColor keyword

[meyfa]

From https://svgwg.org/svg2-draft/painting.html#SpecifyingPaint:

For any value, all color syntaxes defined in CSS Color Module Level 3 must be supported, including rgb(), rgba(), hsl(), hsla(), the extended color keywords and the currentColor value.

See also https://www.w3.org/TR/css-color-3/#currentcolor.