How do I apply opacity to a CSS color variable?

I am designing an app in electron, so I have access to CSS variables. I have defined a color variable in vars.css:

:root {
  --color: #f0f0f0;
}

I want to use this color in main.css, but with some opacity applied:

#element {
  background: (somehow use var(--color) at some opacity);
}

How would I go about doing this? I am not using any preprocessor, only CSS. I would prefer an all-CSS answer, but I will accept JavaScript/jQuery.

I cannot use opacity because I am using a background image that should not be transparent.