The styled mode theming has been reimplemented from scratch based on an all-new architecture. There is no theme.css involved, so you need to migrate to the API. Currently, Aura is the only available built-in preset; however, by v4 final,
two more presets named Lara and Nora would be included, as well as the Material Design theme. If you had a custom theme for v3, the theme needs to be recreated using the new APIs. See the customization section at