@Cybersnaby: Your css looks fine but I doubt about the image URL. Where are you adding this custom CSS and where is you grass.jpg image. I hope you are not editing core theme files inside ‘catch-mustang’ directory. Please don’t edit those files inside core theme directory as these files will be reverted back to original when you update the theme.
So, the image url has to be in reference with the file path. For any css edits either you need to add custom css in “Appearance => Theme Options => Custom CSS” box or you need to add it by building child theme style.css file