You can use regular CSS to custom color your theme. It requires some advanced knowledge. Also theme is not designed for custom colors, it will require more work and attention than regular one line color change codes.
Here is an example CSS code to change header background to blue and footer to gray. You can use this code as it is or edit it to fit your needs.
Paste this code to “Appearance” -> “Theme settings” in osclass admin (when minimalist theme is active).