My background won’t show up!

I made my first theme, and I just can’t get the background code to show up for some reason.

I’ve tried a bunch of php codes, but I don’t know what I’m doing wrong.

Here is the entire Imagine theme zip file for download from my website:
https://seacilly.net/imagine.zip

Here is the image on how I want my website to look:
https://seacilly.net/Picture%202.png

And this is how it looks now:
https://seacilly.net

Thanks!

Please feel free to ask me to supply you with anything that will make it easier to answer this question. It is my first time after all ??