You shouldn’t have two files named style.css however you could rename the old style.css to old.css and import it into the buddypress child of bp default theme.
To create a child of bp-default theme, create a new directory in your themes subfolder ie: /wp-content/themes/shinshan/ and in it put a simple text file named style.css
At the very top of style.css put something like the following:
/*
Theme Name: shinshan
Theme URI: http://example.org/themes/shinshan/
Description: shinshan theme for BuddyPress.
Version: 1.0
Author: shinshan
Author URI: http://example.org/
Template: bp-default
Tags: buddypress, two-column, grey, dark
*/
You could copy and paste the css (without the Theme head section) from your old theme immediately below that header or use the following line:
@import url( old.css );
(presuming the above file is renamed and in the current folder). Then preview or activate your new bp-default child theme.
I suspect that importing the css might cause more problems than it solves – but it depends on what you have in it….
To find out what needs to be changed, webpages can be examined with firebug (download) in firefox, or developer tools in internet exlorer 8/9 and Chrome (press f12 on windows)…