EthicalAds: split `.ethical-alabaster` CSS class into `.ethical-light-theme`

[humitos]

We should not be using .ethical-alabaster for documentation tools that are not Sphinx and are not Alabaster theme. However, as they were looking better than the default style, we started used them there.

We should create a new CSS class for these cases.

• Bring CSS styles from Read the Docs addons into ad client ethical-ad-client#193
• EthicalAd: specific placement for Antora and mdBook #475 (comment)