[Feature Request] Setting stylesheet from file

There's already the option ```vim.g.mkdp_markdown_css``` to set custom css stylesheets globally but it would be really convenient to be able to set it for each file separately.

I already tried
```html

```
in vain (I'm not familiar with how markdown servers work internally)

The themes I set via ```vim.g.mkdp_markdown_css``` also tend to be altered somehow (no background behind code blocks...)