Tiny dialog styles vs. Backdrop dialog styles

[indigoxela]

We have two completely different dialog types. Those created via TinyMCE API and those provided from Backdrop (jQuery.ui actually, but styled differently).

They'll never look the same, but could be more similar.

The tricky part: Backdrop themes set different styles, Tiny skins set different styles... 🤪 A theme could override everything to let them look the same, but a module can't do that.
But we could prepare it a bit...

[indigoxela]

Reopening this, as adding to body isn't ideal - especially the way it's done.

• There may be multiple editors on page (which would add the css class multiple times)
• They may have different skin settings (which makes the css class useless)

On the other hand, if adding to some closer parent element, the class gets pointless for dialog theming...