You can use xprofile fields to specify certain CSS values in the users profile. Then you can retrieve the field value in the profile and modify the profile CSS with jquery. Its a bit of a work around but it works for me. Here’s an example:
1. Create an xprofile field and name it ‘Profile Background’
2. Edit your profile in buddypress and insert ‘#000000′
3. Go to wp-content/themes/yourtheme/members/single/member-header.php
4. On the 2nd line, just after <?php do_action( 'bp_before_member_header' ) ?> paste the following code:
<script>
$("#wrapper").css("background-color", "<?php bp_profile_field_data( 'field=Profile Background' ); ?>");
</script>
This will modify the profile background color according to what ever value you put in ‘Profile Background’
This is just off the top of my head, I’ll check how I’ve done it once I’m home this evening. I will be writing a plugin for this. The only drawback is the user needs javascript enabled other wise its pointless.