Fix CSS overrides with non-default CSS settings

Fixes a bug that occurs when there is a non-default `css` setting specified and you try to preview a markdown file with an extension other than the first listed in `markdown_filetypes` setting (e.g. ".markdown" instead of ".md").

In that particular set of circumstances the `css_filename` variable is referenced in the `if (os.path.isfile(css_filename)):` without it ever being assigned.