Why is this annotation outdated? and shouldn’t it be used? I understand that it is recommended to install the theme using Java code?

It’s not recommended to use @CssImport for an application because you have an Application theme which is easier to maintain. Application Theme | Styling | Vaadin Docs

@CssImport should be primary used for external component package as a jar

@CssImport was useful to change the css inside shadow dom because it was not possible with the webstandard and now it’s possible with ::part, … that is now supported by the newer versions of the browsers

(and you can still target the shadow DOM through the theme folder as well)

thank you