`color()` is not converted to `color-mod()` in variables css files

**Bug**

As the `variables.m.css` and `variables.css` files go through the `postcss-import` plugin, they no longer go through the existing custom plugin that converts usages of `color()` functions to `color-mod()` functions (as supported by the `postcss-preset-env` plugin).

We should also convert the usages of the `color()` functions in variables css files.