Как задать цвет заднего фона для каждой колонки?

class Columns {
    cols = []
    constructor(str, count) {
        this.count = count
        const symbolMax = Math.round(str.length / count)
        for (let i = 0; i < count; i++) {
            this.cols.push(str.substr(i*symbolMax, symbolMax))
        }
    }
    
    /** @param {HTMLElement} container*/
    mount(container) {
        const divs = this.cols.map(i => {
            const container = document.createElement('div')
            container.innerHTML = i
            return container
        })
        container.append(...divs)
    }
}
const col = new Columns(
    '1234567890qwertyuiopZXCVBNMLKJ',
    3
)
const block = document.querySelector('.columns')
col.mount(block)

.columns {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: 10%;
  div {
    background: pink;
  }
  div:first-child {
    background: red;
  }
  
  div:last-child {
    background: green;
  }
}