I wanted to style the course Images using CSS filters, which did not work, because they are set as css Background rather than [img]. Is there another way to accomplish a tint style to course Images, like putting a transparent color layer on top of those containers?
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?