For me a userstyle.css file didn't change the background color of the sidebar.
I figured out, that I am able to change the sidebars background color, by creating a userall.css file and putting it to the location .../dokuwiki/conf/userall.css
.
I wrote the content of the userall.css file like this:
#dokuwiki__aside div{
background-color:rgb(0,0,255);
}
Change the backgroud-color to whatever you like. (In this example it's blue)
And make sure that you clear your webbrowsers cache and then reload the site, otherwise you won't be able to see any changes.
If you don't know how to clear the cache for you specific webbrowser, simply google it.