Using @import caused styles to double up on elements, in some cases preventing the correct override behaviour.
Usint @use instead fixes this, and is recommended by Sass for exactly this reason.
Adds a sass file based on the v10.0.0 Shepherd CSS. Original Shepherd styles are kept where appropriate, otherwise this is intended to inherit whatever styles are being used through the Bulma and Bookwyrm SASS, so that it uses appropriate colours in both light and dark modes.