None of the builtin themes customize the appearance of the `autocomplete` dialog.

However if we want to allow themes to customize the appearance of the dialog, we need `this.popup.setTheme(editor.getTheme());`, this allows themes to customize the autocomplete dialog independently of the editor itself with the `.ace-THEME_NAME.ace_autocomplete` (for example `.ace-ambiance.ace_autocomplete`).

This provides more freedom, and just makes sense (the autocomplete dialog should be abled to be themed like any other part of `ace`).

I'll upate the `ambiance` theme to counter the shadow regression this causes.