With LWC OSS on Node, we could have lwc-services.config.js something like this :

module.exports = {
    resources: [
        { from: 'src/resources/', to: 'dist/resources/' },
        { from: 'node_modules/@salesforce-ux/design-system/assets', to: 'dist/SLDS'}
    ]
};

which copied SLDS assets to dist at build time.

Then index.html included a link to the SLDS CSS :

  <link rel="stylesheet" type="text/css" href="SLDS/styles/salesforce-lightning-design-system.css" />

How should we do this in an LWR app ?