Changing a color background to transparent

[lilacsinrain]

I have tried 5 different themes, but when I try to change the background image, it only shows the default color of that theme.  I can change the header to the image I want, but nothing else.  I haven't found a way to change the color to transparent, and need idiot-proof instructions for that.  I tried 6 different tutorials, but the only advanced code I'm seeing is for the specific color code, not the entire layout code. Any help would be so appreciated, thanks.