Simplenews fails to override the Slate admin theme. See #374222: Template files do not work when using admin theme
Newsletters should be themed using the site default theme. A theme() function is called to render the newsletter content when the node is saved. Normally this theme() function uses the active theme, being the admin theme. To override an existing admin theme simplenews will temporarily clear the $custom_theme value which causes init_theme() to use the default template (patch #59 in the above issue). However Admin module takes over the theming and a simple override by clearing $custom_theme is no longer available.
Does Admin theme provide an other way of temporary override or can this be build in?