Styling: SASS functionality not utilised

### **Enhancement, (Non-urgent)**

**Issue:**
- All SCSS files are written in CSS format. No nesting, variables, includes or mixins are used. The efficiency, simplicity and readability of these files could be vastly improved by harnessing the tools that SASS makes available.

**Proposed Action:**
- Review each individual SASS file, and determine how these these files could be modified to improve efficiency (eg. nesting)
- Review rendered JSX, and determine whether SASS variables could be used (where structure/elements/styling are consistent), and make appropriate changes
- Modify every SASS file to utilise any and every SASS tool available given current CSS-style structure

Review each SASS file alongside the rendered JSX, starting by cataloguing the existing CSS-style structure and repeated values. Determine which nesting, variables, includes, and mixins are appropriate without changing the rendered styling. Done means every SASS file has been reviewed and the resulting styles remain consistent while making meaningful use of SASS functionality.

Written by the indexing model from the issue text.