You can add your own custom HTML but be warned this is an advanced technique and to avoid the risk of someone accidentally breaking their website we've removed the ability to add some types of code.
To add or customise HTML for a particular Bloc find the one you want to edit and highlight some text. This will reveal some further options and will look like this:
To see the HTML click the icon to the far right that looks like: </>
Edit your code as required, remembering to save and/or publish to make the code live on your website.