[Theme: Twenty Fifteen (child)] Sticky sidebar content?

https://garyhollandcomedy.com

I’m running a Twenty Fifteen child theme and the side bar background image is sticky/static in that it doesn’t change when you scroll, but its content (site header, tagline, widgets, menus) do move. How can I make the content stay put too?

Further to this, I’d like to control exactly where the content sits relative to the sidebar. When I’m at the top of the page, the content is lower than I would like.

Thank you in advance for any advice!

Gary