[css-shadow-parts-1] Add a note on the future of theme.

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -270,7 +270,10 @@ Exposing More Widely: the <{html-global/theme}> attribute {#theme-attr}
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 Issue: The theme section is intentionally a rough sketch at the moment.
-It should contain all the details necessary to evaluate the proposal.
+The functionality of theme can be achieved with part alone
+however it may be tedious to explicitly export parts as a theme.
+The need for theme will be better understood once part is in use.
+
 
 In addition to the [=shadow part map=],
 every [=shadow root=] has a <dfn export for="shadow root">partial shadow theme map</dfn>