Right now, you can use the Codex design tokens in MediaWiki, but the import path is annoying. You need to do something like this:

@import ( reference ) '../../../lib/codex-design-tokens/theme-wikimedia-ui.less';

with the number of ../s depending on how deep in the directory tree your file is (since the tokens file is in resources/lib/codex-design-tokens/).

We should explore making this easier, perhaps by adding resources/lib/codex-design-tokens to the importDirs in ResourceLoader::getLessCompiler().