This sounds like a CSS specificity issue. This article has many useful explanations for the subject.
If you wrote the same rule in your external style sheet twice, than the lower rule in your style sheet closer to the element to be styled is considered more specific and therefore will be applied. for example In the following case, padding will be set to 10px, not 5px.
#content h1 { padding: 5px; } #content h1 { padding: 10px; }
To fix your current problem, either, as Dipaks suggested, add your css directly to the page (as this would be preferable to external css files), or even better, or simply put, just add a link to your css file after the Sharepoint css link, and in this case, if they have the same specificity, your css will be applied.
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