How to use theme templates files of non-default theme

I use mailsystem in combination with mimemail and simplenews. In the mailsystem settings, a dedicated theme is selected.
The newsletter content type has an autocomplete entity reference field where the user can select news items.

Only the mimemail template file of de dedicated theme is used. Al the other content (the referenced news items) are rendered via the default theme.

Is there a way to fix this issue?