Markdown was built as a convient set of shortcuts for the verry complex and flexable display language of HTML. Letting other users display and execute unfiltered, user-generated HTML is disabled on almost all websites for security reasons. If we were to add a new intended way of generating colored text, it would take time for projects to implement that feture.
Wait, I didn’t mean accent colour in texts but the ones you can select in the appearance page, which changes components and icons colors. That wouldn’t require HTML injection nor markdown, would it?
So a theme for the whole site?
So, a project like this using stylus? Its CSS made by others injected into the page so that you can make the site look pretty.