Custom style sheets

Would it be possible to add an option to override specific CSS rules in a chosen theme?

For example, I’d like to be able to specify a maximum width for images, while using the default GitHub theme. A field in the advanced options, where I could enter my own CSS, would allow me to achieve this:

.markdown-body img {
    max-width: 20em;
}

The .markdown-body class selector would be required to override the specificity of the default stylesheet (which is annoying and confusing), however just appending this CSS to the end of the selected theme style sheet would be all that’s required to get it working.

[simov]

No, I actually abandoned it due to multiple issues with it. For now you can use other extensions in combination with Markdown Viewer to modify the rendered output.

[JonathanHolvey]

I found that placing a <style> tag at the end of the document was a simple solution to this. Previously I tried putting it at the top, which causes the Markdown parsing to fail.