BuddyPress.org

A little more info please.

Assuming you are using the bp-default theme?

The header image is manageable from the dashboard by the user thus the styles are added to the doc head (embedded styles) these would overrule the rules you create in a linked stylesheet, that’s the first C in CSS ‘Cascading’.

In your functions.php file add this line between php script tags:

/* Disable default themes header stuff */
define( ‘BP_DTHEME_DISABLE_CUSTOM_HEADER’, true );