Change colour of border blocks?

[gnrx]

Thanks snakes1100,
Yes, in the editor variables, I need edit this style var: sidebar_background
I can set background to transparent for sidebar (the blocks at right of forum) but then, I view that, all blocks / sidebars (like My profile) also, appears transparent...jejee. and I only want change this colour in the sidebar (blocks in right of index forum).

Again, thank you!