Add 'system' color support

test color-prop-04-t-manual.svg
```
<rect x="-230" y="-170" width="460" height="300" fill="Background"/>
      <rect x="-220" y="-160" width="440" height="280" fill="AppWorkspace"/>
      <rect x="-152" y="-108" width="317" height="221" fill="Window"/>
```