Update documentation-theme-light to new theme system

I am trying to use https://github.com/documentationjs/documentation-theme-light with the latest code for documentationjs like below.

```
./node_modules/.bin/documentation \
build \
src/*.js\
-f html \
-t node_modules/documentation-theme-default \
--name test
```

and I keep getting error

```
docs/node_modules/documentation/lib/commands/build.js:60
throw err;
^

TypeError: theme is not a function
at Object.makeHTML [as html] (docs/node_modules/documentation/lib/output/html.js:21:3)
at onDocumented (docs/node_modules/documentation/lib/commands/build.js:63:47)
at docs/node_modules/documentation/index.js:101:7
at docs/node_modules/documentation/lib/input/dependency.js:45:5
at ConcatStream. (docs/node_modules/documentation/node_modules/concat-stream/index.js:36:43)
at emitNone (events.js:72:20)
at ConcatStream.emit (events.js:166:7)
at finishMaybe (docs/node_modules/documentation/node_modules/concat-stream/node_modules/readable-stream/lib/_stream_writable.js:475:14)
at endWritable (docs/node_modules/documentation/node_modules/concat-stream/node_modules/readable-stream/lib/_stream_writable.js:485:3)
at ConcatStream.Writable.end (docs/node_modules/documentation/node_modules/concat-stream/node_modules/readable-stream/lib/_stream_writable.js:455:41)
```