You will need to know CSS to make changes such a colors, and know how to target to specific area you want to change. I suggest using firefox and firebug. Below I have attempted to show you what you can change and can not change as far as starting a child theme style.css. This commented section has to be included in your child themes folders style.css and the folder with the style.css needs to be placed within wp-content/themes and you have to activate it. If its left blank it just inherits the bp-default style.css. If you correctly target what you want to change then this child themes style.css will over-ride the bp-default style.css. Whatever you target and decide you want to change needs to be placed after the closing comment ( */ ) of the the below code with proper CSS syntax. I hope this in some way explains it a little more clearly.
`/*
Theme Name: BuddyPress Dusk /*This can be changed*/
Theme URI: http://example.org/themes/dusk/ /*This can be changed*/
Description: Dark theme for BuddyPress. /*This can be changed*/
Version: 1.0 /*This can be changed*/
Author: John Doe /*This can be changed*/
Author URI: http://example.org/ /*This can be changed*/
Template: bp-default /*This CAN NOT be changed*/
Tags: buddypress, two-column, grey, dark /*You need to have buddypress in the Tags, the rest (example: two-column, grey, dark can change) */
*/`
