Add note for importing in sub folders.

Addresses vaadin/flow#9794

articles/theming/application-theme.asciidoc

@@ -77,6 +77,9 @@ See <<styling-applications#, Styling Applications>>.
 NOTE: Global styles can override <<theme-variants#global-variants, global variants>> styles.
 For example, `Lumo.DARK` variant defines a `--lumo-base-color` which can be overridden in CSS files.
 
+NOTE: At the moment any CSS imports need to be applied from files on the root level.
+See Vaadin Flow https://github.com/vaadin/flow/issues/9794[issue 9794]
+
 === Component Styles
 
 Component-specific style sheets should be placed under the `components` folder.
@@ -362,4 +365,5 @@ The following options are not available:
 <2> Switching between application themes at runtime.
 <3> Defining multiple parent themes.
 <4> Having a theme based on Material built-in theme.
+<5> Using imports in theme sub folders
 Application themes can be only based on <<overview#built-in-themes,Lumo>> built-in theme.