[TYPO3] CSS styles only working with tt_content?!

[Kanu- und Ski-Club Lünen e.V. - Thorsten Dorn]

Sorry, but I do not understand this;

> as default plugins set embedded style sheets, which override linked
> style sheets if they are belo the link tag. These style sheets must
> disable, for example
>
>
> plugin.tt_news._CSS_DEFAULT_STYLE >
> plugin.tx_veguestbook_pi1._CSS_DEFAULT_STYLE >
>
> If you disable all of them you can use an external style sheet  
> defining
> all CSS.

Could you please explain this in more detail?