What you could do with is another layer over the course image. I dare say it could be added using a renderer. Then all you would need to do is add a background rgba() color using CSS which i tend to use in my page backgrounds on my Moodle Test site.
I did something similar in the Morecandy theme Banner where I had a setting that allowed a texturized image that added a distinct colored texture to the banner.
Hope this helps?