Want to use your own custom css ?
No problem! Please use this code snippet
https://gist.github.com/1034853
I don't want to change the page background, rather I want to change the background of profiles wrapper div.
Well, It mostly depends on your theme. here is an example for how to do that with bp-default theme.
https://gist.github.com/1034872
You will need to adjust that for your current theme.