using picture as a background image on touchfolio theme

Hi,

Using theme touchfolio: https://dimsemenov.com/themes/touchfolio/about.html

So i wanted to use background image, on sidebar area:

Theme css looks like that:

////////////////////////////////////////////
//
// 3. Background colors
//
////////////////////////////////////////////

@content-background: #F8F8F6;
@sidebar-background: #FFFFFF;

I dont know much about coding, so i tried to change:
@sidebar-background: #FFFFFF;

to something like that:
@sidebar-background: #FFFFFF url(/image.png);

Image showed up on the area after content area (on footer, which is good), but after doing that it also totally removed sidebar area and only shows content area on my page.

Any ideas?

Thanks in advance.