fix/456-default-value-ignored-dark-mode

[orhels]

fixes #456

• Verify that variableValue['@media (prefers-color-scheme: dark)'] is defined before using it as defaultValue
• Added test for css.defineVars

This fix aligns the behavior across light and dark mode. A better solution would probably be to actually support a subset of the media query specification, but I think that would require some more effort and planing.

The tests are a little strange as they tests that using media-queries other than @media (prefers-color-scheme: dark) will return the default value, and not that other media queries actually works. I used the css-create-queries-test.native.js as a template and tried to keep the test names a concise as possible.

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