Markdown custom themes

The problem might lie with my limited understanding of CSS. For instance in a markdown editor there will always be a folder where you can drop a .css file and change the styling of your text (markdown or dropped from html). It seems that you can use the same style sheet across all markdown editors and the styling is exactly the same. You edit your text in markdown format and preview your markdown with the .css you’ve used. That could be rendered as HTML, pdf, etc.
Now, in tinderbox, I learned that markdown is supported then I saw that the markdown text is styled with a .css file. Hold and behold, it also has a preview pane. Conceptually, to my intuitive (intuitive nonetheless) reasoning, this seems to work on the same principle as a a markdown editor. Markdown in text pane and style sheet in preview pane. Again, I’m very limited to the nuances of Tinderbox and more so when it comes to CSS. I believe that I am not understanding The difference (under the hood that is) between a markdown editor and Tinderbox and how they operate in rendering markdown and styling their text.
Is the css file language syntax used in a markdown the same as the one used in tinderbox? I think that would be a start for this challenge. Regardless I will try all the steps again. Thanks a bunch for the input.