Readability - overriding user's prefered font

[StephDriver]

Font often affects readability. In A11y terms, some users will have preferred fonts which they find easier to read and have them set up as defaults in their own browser.

From (#4194)

• The OLH theme puts their preferred font as the main article font.
• The clean theme only uses it for the word 'contact' next to the envelope icon by the author's name.
• Not found within the material theme.

Consider how the user's default fonts can be honoured - and be consistent within a theme!

[StephDriver]

• The clean theme only uses it for the word 'contact' next to the envelope icon by the author's name. This text needs to be styled just like the rest so that this doesn't happen.

[StephDriver]

could be combined with work on

• Dyslexia Features across themes #755
  which will need fonts standardised.