Use a Sass variable for default font

[Bisaloo]

The goal of this PR is to facilitate theming in varnish forks.

Currently, forks that want to set their own fonts need to edit numerous files and it takes a lot of maintainance work to fix the various git conflicts that arise as a result when pulling from upstream.

By setting the font once as a Sass variable, this should reduce the difficulty of:

• creating a custom theme using custom fonts
• keeping this theme up to date with the active development of varnish by making it easier to rebase / merge with upstream

I'm definitely not a CSS or Sass expert but I have tested this locally and it works as expected as far as I can tell.

[froggleston]

Hey @Bisaloo - please could you merge in main/rebase this PR and we can move forward with it?

[froggleston]

LGTM! Thank you @Bisaloo !