BuddyPress.org

We can’t help without Live example to view the source..

For CSS – Add the following to your theme stylesheet and change it to whatever color you want

#buddypress a.activity-time-since {
	color: #aaa;
	text-decoration: none;
}
#buddypress a.activity-time-since:hover {
	color: #888;
	text-decoration: underline;
}

The “1 day, 1 hour ago” text is coming from \plugins\buddypress\bp-activity\bp-activity-template.php line 1332
$time_since = apply_filters_ref_array( 'bp_activity_time_since', array( '<span class="time-since">' . bp_core_time_since( $activities_template->activity->date_recorded ) . '</span>', &$activities_template->activity ) );

However, It’s not recommended to change core files. And I have no idea how to do it externally. Hope this helps