Customize > Additional CSS not applied to the editor's preview pane

[SorinGFS]

Describe the bug Since there is no custom.css allowing the user to customize the style the only way to do it is to customize the styles in the Customize > Additional CSS tab. So far so good, but there is a problem: the customized style does not appear in the Editor's Preview pane, so the user can't use the Preview pane to see the changes, it must hit update and go to View Page to see the changes.

To Reproduce Put this in Customize > Additional CSS:

body {
    color: red;
}

Expected behavior The text color in Editor's Preview should now be red!

Solution Well, there are a couple:

• create a new stylesheet called custom.css and allow the user to make the changes in there... or
• apply the styles inserted in Customize > Additional CSS to the Editor's Preview pane too.

[SorinGFS]

And one more thing, this is not the only difference between what we see in Editor's Preview versus Page View! Some things are working in editor and not working in page, and some things are not working in editor... but working in page!

Feature	Working in Editor	Working in Page
Math Formulas	yes	no
Mermaid diagrams	no	yes
nested lists differentiated bullets	no	yes
choosen highlight theme	no	yes