Modify CSS variable in media query - css

Question

Is there a way to modify a previously-declared CSS variable inside a media query with just vanilla CSS? What I'm after would look like this (which of course doesn't work as the variables all get computed in the end):
#container {
--elem-size: 20px;
}
#media only screen and (min-width: 800px) {
#container {
--elem-size: calc(var(--elem-size) * 2);
}
}
I'm aware that it would be possible to declare a "base variable" (e.g. --base-elem-size) and then use it to generate new variable values for different viewports. However, I'm working with a very large number of CSS variables which makes it undesirable to create a duplicate base set out of them. The ideal solution would be able to "modify" a previously-declared value.

Answer 1

This might be far from ideal but to some degrees does define the style in one line. It uses fallback values of CSS variables.
Open full screen and change window size to see the result:
:root {
--elem-size-base: 50px;
}
#container {
/* 👇 Fallback for base value, optional 2nd fallback for no variable value */
width: var(--elem-size, var(--elem-size-base, 50px));
height: var(--elem-size, var(--elem-size-base, 50px));
outline: 2px solid #000;
background-color: pink;
}
#media only screen and (min-width: 768px) {
#container {
/* 👇 This variable for various #media */
--elem-size: calc(var(--elem-size-base) * 2);
background-color: lightblue;
}
}
#media only screen and (min-width: 1024px) {
#container {
/* 👇 This variable for various #media */
--elem-size: calc(var(--elem-size-base) * 4);
background-color: lightgreen;
}
}
<div id="container"></div>

Answer 2

I think having base CSS variables would make your CSS much more readable and easier to maintain, specially if you are working with a big codebase.
May be something like this:
:root {
/* Define you base variables */
--font-size-default: 16px;
--font-size--large: calc(var(--font-size-default) * 1.25);
/* Assign the base variable to the element variables */
--elem-size: var(--font-size-default);
}
#media screen and (min-width: 840px) {
:root {
/* Change base variable used for element variable */
--elem-size: var(--font-size--large);
}
}
This allows you to set a base font-size variable and one (or more) "variants" of that variable, which you can then use on your whole project
I would suggest you look into something like the Open Props (which has some really great ideas for what base variables can be and how to use them)