Support generating pictures with light background

Support `color-mode:light` when generating pictures with `polyx`, e.g,.: 
```
clojure -M:polyx info color-mode:light out:info-light.png
```

If we want a transparent background, we can type:
```
clojure -M:polyx info out:info-transparent.png :transparent
```

I needed this functionality myself, if you wonder why this was prioritized! :-)