Are you referring to using CSS in an email client or a web browser? If the former, CSS in emails is usually quite tricky to do because support is even worse than it is for the web. If it's the later, you could try using the "!important" declaration in your external CSS, but that probably won't help. Remember the precedence for CSS...

@import CSS is overridden by...
<link> import CSS which is overridden by...
<style> CSS which is overridden by...
inline CSS

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