Fix Sass support for styles.css

[SilasBerger]

Sass support works fine in all components, with the docusaurus-plugin-sass and sass dependencies, the 'docusaurus-plugin-sass' plugin entry in docusaurus.config.ts, and the "types": ["docusaurus-plugin-sass"]addition intsconfig.json`.

However, Sass does not seem to take effect on src/css/styles.scss (for instance, variables are not resolved but used as plain strings instead). The file is currently renamed back to styles.css, and the customCss: ... entry in docusaurus.config.ts adjusted accordingly.