Yes, the simplest way would be to insert them as static elements, if you want to be able to do this via the editor you'd need to write a tool to support the service you wanted to insert content for (e.g gist) or a general purpose insert static content tool (neither or which is a huge challenge).
Yes, the easiest way is to modify the CSS, however ContentTools itself is just a base editor with a small UI library, many people have used it as a basis for their own editors but fundamentally changed all of the UI (for example https://en-gb.wordpress.org/plugins/page-builder-sandwich/).
I want to know two things:
Thank you for your work.