Consistent style is not enforced in SCSS files

[alanorth]

Is your feature request related to a problem? Please describe.
We do not currently enforce any code style in SCSS files. This leads to an inconsistent use of indentation, spacing, and SCSS syntax, which makes it frustrating to compare component styles in themes with those in src/app as well as with each other (for example the custom and dspace themes).

Describe the solution you'd like
We should be enforcing a consistent code style in SCSS files with Prettier and or Stylelint. See this comparison from the Prettier documentation.

Further rationale: if we use an opinionated, automated formatter we can outsource that work to the tool and just forget about it. In the golang community there is gofmt and it is commonly said that:

Gofmt's style is nobody's favourite, but gofmt is everybody's favourite.

Describe alternatives or workarounds you've considered
None.

Additional context
We use checkstyle for Java code in the backend and eslint for TypeScript code in the frontend.