fix: [#1363] Fix bug with missing fallback value

[odanado]

Close #1363

Custom properties without fallback values can be converted to custom properties with fallback values (using SINGLE_CSS_VARIABLE_REGEXP).

e.g.

• var(--known)
  • In this case, you can resolve the value of the custom property by examining the value of --known.
• var(--unknown, var(--known)
  • This case can be converted to a custom property with a fallback value of the form var(--unknown, known-color) by resolving the --known value.
• var(--unknown, var(--unknown, var(--known)))
  • This case can also be converted to a custom property with a fallback value of the form var(--unknown, var(--unknown, known-color)) by resolving the value of --known.

After converting to a custom property for which a fallback value exists, the value can be found using a regular expression that handles the custom property in this case (using CSS_VARIABLE_REGEXP).

[capricorn86]

Thank you for your contribution @odanado! ⭐

I simplified the solution a bit by fixing the original RegExp, which will only require one white loop.