Lumo should just load stylesheets in V25

[rolfsmeds]

Describe your motivation

Currently, applying a theme (in practice Lumo or Material) rewrites components' JSModule paths to the theme-specific module.

As of V25, Material is removed, and the Lumo styles for components are no longer loaded as part of the component dependencies but as regular stylesheets loaded separately from the components themselves: @CssImport("@vaadin/vaadin-lumo-styles/lumo.css") or, in css, @import("@vaadin/vaadin-lumo-styles/lumo.css").

The @Theme annotation is retained for backward compatibility, but only as a shorthand for loading the Lumo stylesheet, and, if specified, the application theme.

Describe the solution you'd like

• @Theme(themeClass=Lumo.class) should just do a @CssImport("@vaadin/vaadin-lumo-styles/lumo.css")
• @Theme("mytheme") should do the import and then load the specified folder's styles.css
• @Theme(themeClass=Lumo.class, variant=Lumo.DARK) should do the import and apply the theme="dark" to the page (as it does today).

Perhaps this could be implemented simply by a @CssImport in the Lumo theme class, instead of the current lumo-styles npm package import?

Additionally, the Lumo theme class should no longer load various JSModules as these will no longer exist.