Styled mode svg export while using as server, no css applied at all

#### Expected behaviour
When i add css resource or inject css using customCode the output svg has the correct styles applied.
Only when i change the type to png is working at the moment.

#### Actual behaviour
Everthing is black

#### Reproduction steps

Use http request
```
{
"type": "svg",
"constr": "chart",
"width": 0,
"height": 0,
"allowFileResources": true,
"allowCodeExecution": true,
"customCode":"function() { let head = document.getElementsByTagName('HEAD')[0]; let link = document.createElement('link'); link.rel = 'stylesheet'; link.type = 'text/css'; link.href = 'http://localhost:8000/highcharts.css'; head.appendChild(link);}"
,
"resources":{
"css":".highcharts-background { fill: red}"
},
"infile": {
"chart": {
"styledMode": false
},
"series": [
{
"data": [
1,
3,
2,
4
]
},
{
"data": [
324,
124,
547,
221
]
}
]
}
}
```