the only valid URL to theme customizer is: ../wp-admin/customize.php?return=%2Fwp-admin%2Fthemes.php
BuddyPress pages are not customizable like ordinary WP pages. BP pages have dynamic content and can be adjusted to any theme directly by CSS and a child theme. These page should only exist with a title, and nothing else.
Depending the theme you use, and his complexity, customize it to your need first. Once your WP works with the desired layout, you activate BuddyPress.
This should work flawless, out of the box generaly. If you estimate that some details, like a header width or an avatar size should be modified, you do that directly via CSS.
Read codex for more details about themes.