@elin, There is a couple things going on here. First there is some bug with this plugin that I have not been able to track down however there is a work around for. When you set the background color in the plugin it will not apply unless you have an image in use. So the work around I have used is you can create a transparent 1px PNG file and set the plugin to not repeat the background image. This will tell the plugin to activate and set the background color (but using a single pixel transparent image).
Now as for your theme, I see that the background-4.png is not being applied to the BODY tag but is actually using a DIV (ID name “wrapper”) that is acting as the body. This plugin when selecting “body” uses the BODY tag and applies the background to that. This means that your DIV is on top of the BODY there for it will be hidden under the DIV. In order to correct this you will need to edit the plugin to use the “wrapper” DIV tag instead of the body. I see in your code that you have set the “wrapper” background to be !important.
I am sorry but I am not familiar with the wootheme Athena in how it is setup to let people customize, so I am not sure what/where/why this was set this way to make this happen. However if you must use the !important tag, you can edit the plugin to apply this as well. The only only think to make sure is that the plugin applies your new background settings underneath the custom “wrapper” settings you have so that it will be set !important.
Sorry for the long post. I hope I am clear on what I am pointing out. Also I can point out where in the plugin code you need to change if needed.