PDF: Tweak font-size and colors to more closely match other MD WebUI
MD PDF was written before we had a firm style for MD WebUI, both in
terms of visual style (typography, etc) and code style. This CL updates
MD PDF with a few changes to bring it in line with more recent changes.
* Load the MD version of text_defaults.css, changing how fonts are
applied across the page.
* Update default text to 13px paper-grey-900. This only affects the
bookmarks dropdown and error/password dialogs.
* Tweak padding in the main toolbar.
* Remove usage of paper-material.
* Change how CSS variables are applied to the page (using a custom-style