Document how to create a custom theme

[tobyhodges]

When carpentries/sandpaper#585 is merged, the documentation for this package at https://carpentries.github.io/varnish/ could be updated to include a section (or a separate vignette?) describing how to create a custom theme from varnish for Workbench sites.

• using a custom logo
• adjusting the theme colours
• (if/when Use lesson program-specific favicons #33 is addressed) adding custom favicons