Improve checkTheme script

[joshgoebel]

See also: #3402 (comment)

We have some themes that use rgba(), will .hex() work on those values?

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Two notes on this one:

1. Some styles use background-color to differentiate the background for specific elements. I think we should add support for checking background-color for WCAG contrast.
   

   highlight.js/src/styles/foundation.css

   Lines 74 to 77 in 0bf3ade

   	.hljs-regexp {
   	background-color: #fff0ff;
   	color: #880088;
   	}

2. Should we have a minimum Node defined somewhere for dev environments? Optional chaining was added in Node 14 and our package.json requires Node 12+ (correctly so, for our users).

Originally posted by @allejo in #3402 (comment)