Steps to reproduce

Open a page in page-editor.

Expected results

Basic HTML elements are not styled.

Actual results

Basic HTML elements are styled.

Workaround

Development notes

In Pages app inside preview iframe Magnolia injects following CSS file:
<link type="text/css" rel="stylesheet" href="/magnoliaAuthor/VAADIN/themes/pages-app/page-editor.css">
Inside file we can find it does not only affect green bars but also some basic elements like headings e.g: 
h1, .heading-1, .h1

and has some very generic named classes e.g.:
text-medium, text-regular
As well as loading Roboto font with again generic font-family name: Roboto

All styles within that file should have Magnolia specific prefix/suffix/name to avoid conflicts with template styles and elements.