Theme parsing improvements

[gcushen]

This issue continues on from where #680 left off.

Todo

• [x] Add more defaults in case user provides incomplete font or color theme
• [ ] Use local newScratch scratch, SetInMap, Hugo's new 0.55 return function, and refactor calls to parse_theme
• [x] Replace primary_light/dark with SCSS lighten/darken funcs
• [ ] Use Bootstrap's $link-hover-color for link_hover
• [ ] Refactor Academic's grey colours as variables (possibly use existing Bootstrap grey vars or use its muted selector more consistently)
• [ ] Refactor to use store defaults in dict and apply Hugo v0.56 Merge

[gcushen]

Closing due to inactivity, lack of contributions, and as this issue is no longer relevant going forward (see #2792).