Are you using a child theme? If not you should create one (http://codex.wordpress.org/Child_Themes) as any edits made to twentyten files will be lost when WP or 2010 is upgraded.
In the style.css look for #wrapper & change the colour from #fff to whatever you want it to be.
eg
#wrapper{background:#efe;}
@ cubecolour:
thank you for your response! Im not using a child theme so Im trying to create one – but I cannot access wp-content/themes,and therefore I can not make a new directory for the child theme?!
I cant find it in wordpress even if I write it in my browsers adressline it wont show. (www.mydomain.com/wp-content/themes)
where do I find my way in to wp-content/themes??
thank you so much!
See the codex page on child themes (linked to above) for the basics on creating the files that make up the child theme.
Then you need to connect via ftp to upload the child theme files you have created.