Customize Markdown View


how can I customize the view of markdown files - both for the (WYSIWYG)-editor as well as just viewing?
I’m using 7.1.4 CE.
Especially, I want column width of tables to be adjusted automatically to the contents of the columns. I understand that somehow this can be done using custom css and an old thread tells me it should be possible:

Unfortunately, my experience with css is very limited (nonexistent). Maybe someone can point me in the right direction.

Any help is greatly appreciated!