There is a constant for this:
plugin.tx_news.settings.cssFile = EXT:news/Resources/Public/Css/news-basic.css
Just overwrite it with the path to your css file in the constants section of your root template like this:
plugin.tx_news.settings.cssFile = path/to/file.css
Or remove the whole css file output by adding that instead:
plugin.tx_news.settings.cssFile >