How can I disable css rule

– I’m a noob to css.
I’m using a theme with a background image, which I like, but I want to disable and swap it for a plain colour in my blog section–it’s a bit choppy when scrolling. I have figured out the rule to add the colour, but I don’t know how to disable the rule leading to the path of the png file.

I have add this into my custom css, but it doesn’t work!!

body.custom-background {
background-image: url(“http://onlineteacher123.com/wp-content/themes/gridiculous/images/solid.png”);
background-repeat: repeat;
background-position: left top;
background-attachment: scroll;
background-color: lightgray;
}

I know that it will work, if I somehow could tell the css console to override this rule
background-image: url(“http://onlineteacher123.com/wp-content/themes/gridiculous/images/solid.png”);

Thanks