Support applying `.likely-light` styling conditionally for dark theme users

[sashotoster]

This way developers that support different page styling based on their end users' light / dark theme preferences can also make use of both our regular & dark themes. Applying .likely-light unconditionally works as before.

Readme is updated accordingly, including the missing information about the already existing likely-light theme (was reflected in the screenshots, but not in the description).

Additionally, the manual testing file is extended with rows having hardcoded / conditional dark theme (doubting about this one, done in a separate easily revertable commit).

[sashotoster]

@NikolayRys added a small readme fix as it's not worth its own PR

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