Fix extracting CSS variables with default value

[hyrious]

There are 2 issues in previous code:

1. Applying CSS variables wrongly handled non-exist variables, resulting undefined at value. It should be left as is.

2. GitHub added 2 new CSS variables with default fallback, previously the script does not handle this case well, it can correctly extract them now.

[sindresorhus]

When generating, I noticed that it now includes some things we don't want, like [data-dev-analytics-enabled]. Would you be able to remove that?

[hyrious]

I'd like to, looks like these unwanted styles are extracted from top level rules (since they may have chance to affect markdown body).

Previously we only have one forbidden rule [hidden][hidden], maybe it's time to disallow more selectors.

Let's print out these selectors to have a glance (omitting '.class', ':root', '::-webkit-whatever', 'p'..):

[data-color-mode=light][data-light-theme*=light]
[data-color-mode=dark][data-dark-theme*=light]
[data-color-mode=light][data-light-theme*=dark]
[data-color-mode=dark][data-dark-theme*=dark]
[data-color-mode]
[hidden]
[type=reset]
[type=submit]
[type=checkbox]
[type=radio]
[type=number]::-webkit-inner-spin-button
[type=number]::-webkit-outer-spin-button
[type=search]::-webkit-search-cancel-button
[type=search]::-webkit-search-decoration
[role=button]:focus
[role=button]:focus:not(:focus-visible)
[role=button]:focus-visible
[data-catalyst]
[data-catalyst-inline]
[data-a11y-animated-images=system] [data-animated-image]
[data-a11y-animated-images=disabled] [data-animated-image]
[data-color-mode=light][data-light-theme*=dark] ::selection
[data-color-mode=dark][data-dark-theme*=dark] ::selection
[data-dev-analytics-enabled] [data-feeds-analytics]
[data-dev-analytics-enabled] [data-feeds-analytics]::after
[data-dev-analytics-enabled] [data-feeds-analytics]:hover
[data-dev-analytics-enabled] [data-feeds-analytics]:hover::after

[sindresorhus]

maybe it's time to disallow more selectors.

👍