[Feature Request] Setting stylesheet from file

[JMans15]

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

<link rel="stylesheet" href="path">

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...)