BuddyPress.org

Are you sure that your custom-sample.css file is actually being included? With a file name like ‘custom-sample’, I’m guessing that it might not be. Check out the theme’s main style.css, and see if there’s an @import rule for custom-sample.css (or if there is a rule in any other imported stylesheet).

I’m not sure how bp-social works (it appears to be a premium theme so I can’t look at the code) but if it is a child of bp-default, then you won’t be able to create a “grandchild” theme so that your changes won’t be overridden – WP doesn’t support that. However, you could fake it by dropping a snippet in your bp-custom.php file that looks something like this:

`function bbg_custom_style() {
$myStyleUrl = WP_PLUGIN_URL . ‘/custom.css’;
$myStyleFile = WP_PLUGIN_DIR . ‘/custom.css’;
if ( file_exists($myStyleFile) ) {
wp_register_style(‘myStyleSheet’, $myStyleUrl);
wp_enqueue_style( ‘myStyleSheet’);
}
}
add_action(‘wp_print_styles’, ‘bbg_custom_style’);`

(See https://codex.wordpress.org/Function_Reference/wp_enqueue_style for more info on how to enqueue styles)

Just make sure that you have your custom styles in a file called custom.css in your plugin directory. That should make sure that your styles get loaded, but that they don’t get overwritten by future upgrades to your main theme.