Hi,
in many or maybe all templates (for example Enlighten) the sidebar background colour will override the main content background colour in small responsive mode. Normally with responsive sites each block keeps its own background, unless specified otherwise in custom CSS.
Also the sidebar content will show above the main content in small responsive mode. Normally, the main content shows first, followed by secondary and other content. This is the other way round, which makes no sense to visitors. It's not in the main content area for a reason. Showing a site on a small screen should not affect content priorities.