Sidebar width and background color

Hello! I am working on a child theme to make some modifications to Twenty Fifteen.

The chief changes I want to make are global changes to:

– Sidebar width. (I want a narrower sidebar)
– Background color. (I’d like to change the background of the entire page, including the sidebar and the post content)
– Background image. (If possible, I’d like the background image to appear behind the sidebar and behind the post content)

Because this theme’s structure is very complex, I’m having trouble making these changes consistently. For instance, sidebar header background color overrides the sidebar background. And setting a fixed width sidebar seems to have unpredictable results at different widths.

The result is that I’m having to create very long lists of enumerated classes that need to be changed. It works, but I’m mostly using trial and error to discover which classes need to be added to each list.

Any guidance would be welcome!