There are a couple of online SaaS tools doing what are you looking for.
It is the tool you are looking for: https://debounce.io/solutions/data-appending-and-enrichment/
You may also check this API: https://debounce.readme.io/reference/data-append
Answering my own question:
Recently I have installed the free NodeJS-based command-line tool
Juice (github, >2.4 k stars)
Given HTML, juice will inline your CSS properties into the style attribute.
Juice generates output that differs only marginally from the output of the MailChimp CSS inliner tool.
Now I use a purely command-line based workflow for ...