Improve checkTheme script

See also: https://github.com/highlightjs/highlight.js/pull/3402#discussion_r765491600

> 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.
https://github.com/highlightjs/highlight.js/blob/0bf3adeecd5ba76cef8ee843e38722089b232da1/src/styles/foundation.css#L74-L77

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 https://github.com/highlightjs/highlight.js/pull/3402#discussion_r765487222_