Index background color

Hi guys, was hoping for some help on this topic. I circled some spots in yellow on the image I attached. I’m trying to set up a color block on a few index spots like the image shows. By using the code I pasted below. This made it possible to color 50% of the index. I was hoping to be more around 25-30% of the page. Is this possible? Thank you, any help is welcome! 

background: linear-gradient(90deg, #ebebeb 50%, #fff 50%); 

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