V3: CSS Variable/Properties Support

[sagalbot]

• [ ] CSS Custom Properties for all scss variables
• [ ] Add https://github.com/postcss/postcss-custom-properties

[KareemDa]

Here is the problem:

• preselected values are inserted as string,
• while when selecting a new file It's inserted to the array as an object

[KareemDa]

we might need to update the behaviour of selectedValue array,

If multiple, label is defined, we should convert the array of strings to array of objects corresponding to this string.