[Feature] Custom Styles does not allow for modification of the Basics section

Is there an existing issue for this feature?

• Yes, I have searched the existing issues and it doesn't exist.

Feature Description

The new Custom Styles feature, while generally a good move (I really like the WYSIWYG nature), seems to have regressed on a number of extremely useful features - namely the ability to really drill down and tweak any part of the document layout.

In particular, I used Custom CSS quite heavily to brand the header to my liking (gradients, font styling, etc.). This has all disappeared into the void as of the latest update and I'm unable to get anywhere near my old layout.

Extending Custom Styles to allow for modification of templated areas (including the header area and the page body) would go a long way to getting me back to something approximating my vision.

This is arguably a regression and as such a bug, but I'm filing as a feature on the basis that it's an extension of the new functionality.