Custom image in Sidebar

Hello and thanks for many great themes.

I am loking to replace the dark/light style control on s2 with a fixed image.

Should be this code
.s2 .social-links {background-image: url("https://via.tt.se/data/images/00807/95e5a5f4-a985-4d42-a12a-2137a674bda5.png);}
The link is to an example image.
Intended result something like this

https://i.imgur.com/ENOOWGY.png

(bad example but you get the idea..)

Not getting it right. Using a child theme css.

The page I need help with: [log in to see the link]