Add support for CSS variable replacement

[Jaeiya]

Before conversion

root: {
  --red: #ff0000;
}
.myclass { color: var(--red); }

After conversion

.myclass { color: #ff0000; }

I have a very large file with all the variables inside it for theme management; it would be nice to remove all those long names and just replace them with the colors. It would save a huge amount of space.

[yawnoc]

CSS variables (custom properties) can be inherited, so there are use cases where replacement is undesirable. E.g. if you define --colour-1, --colour-2, etc. in an external "theme" stylesheet, and then reference these variables in an embedded stylesheet.

There would need to be an option to disable variable replacement. Or even better, let the user supply a whitelist/blacklist of variable names that can/cannot be replaced.

[dqh-au]

It would also break situations when variables are used as runtime variables, for example overriding colours based on dark mode:

:root {
	--fg-color: black;
}
@media (prefers-color-scheme:dark) {
	:root {
		--fg-color: #dcdcdc;
	}
}

It's also possible to manipulate variable values from JavaScript. So I think if implemented this should be disabled by default.