Change checkbox background colour when checked in shiny:checkboxGroupInput()

I've built a shiny app with checkboxGroupInput's that as a default show blue background when checked, e.g.

checkboxGroupInput("water", "Water Level",
          choiceValues = c('Low', 'Medium','High'),
          selected = 'High')

Now, I want to change check box background colour to dark grey when checked (HEX #666666) using CSS within the app.

I tried the following CSS options, with no change to app's look:

tags$style(HTML('

    .checkbox input[type="checkbox"]:checked {
        position: absolute;
        margin-left: -20px;
        box-shadow: #666666;
        background-color: #666666;

section.sidebar .shiny-input-container:checked {
    padding: 12px 15px 0px 15px;
    white-space: normal;
    color: #423F3D;
    box-shadow: #666666;
    background-color: #666666;'
}

Any ideas how to achieve it?