Classic theme: how to add color background

[Pierre_d]

Hi All, I'm changing a bit the Classic theme for my shop. I've already changed the colors of several elements by adding my own CSS to  custom.css, but I would like to do something a bit more complex. But with my limited understanding of CSS and Prestashop theme, I don't even have a clue where to start!

Here is what I've done so far:

And here what I'm trying to do: add a colored background below the category treeview that extends to left side of the browser window.
I'm not asking for the complete solution but if some one could "put me on right track", I would appreciate!

Edited January 7, 2018 by Pierre_d (see edit history)

[Pierre_d]

Forget about it! I found a way, but requires too much changes and testing of .tpl.