I want to change background of github markdown file. How can I do it? Is there specific markdown command to do that or maybe some HTML or CSS code? Or some other way?
As far as I know, there's no way to do that.
In this file you'll find available HTML tags in github-flavored markdown: https://github.com/gjtorikian/html-pipeline/blob/main/lib/html_pipeline/sanitization_filter.rb (this is linked to by Github's README themselves)
It is also said here that:
The HTML is sanitized, aggressively removing things that could harm you and your kin—such as script tags, inline-styles, and class or id attributes.