Custom HTML is not styled

[joyously]

A Custom HTML block is rendered in the editor in an iframe. This means that none of my theme's styles can affect it.

I tried this with WP 5.2.1 and with GB 5.8.0.

To reproduce
Steps to reproduce the behavior:

1. Define a class in your theme's style sheet to change the color.
2. Add a Custom HTML block
3. Enter HTML with the class to change the color.
4. Click on Preview on the Custom HTML block.
5. See that the color is not changed.

Expected behavior
The theme's styles should be able to affect every block, so the Custom HTML should be the changed color.

Screenshots
This shows my test, using a Custom HTML block and then the same HTML in a Paragraph block.

Desktop

• OS: Ubuntu Studio 16.04
• Browser Firefox
• Version 66

[swissspidy]

I believe this is a duplicate of #9129.