There are a couple of variations to easily modify themes. One is at the Block level by selecting "Design" when editing. Also, some add-ons in the marketplace will customize the background.
To do what you want however, open the main css file ( on a download of concrete5.4.1.1 open "concrete", then "themes", then "default" to see the file folders you need. When you view "main" with editing software, search out the color codes for the areas you want to modify (you can Google the color codes to tell whats installed and to find a replacement). Save the edited folder and use FTP software like Filezilla to replace the original folder installed. Refresh your site and... ta-dah A customized theme uniquely yours :)

PS save a copy of changed folders in case you lose changes when you eventually upgrade to a new version of C5 or have to reinstall it.