Lead Developer
Hi there,
Actually, themes can’t affect each other as none of the theme functions are read if the theme isn’t active.
What you’re experiencing with your menus is a default WordPress behavior – when you switch themes, you need to make sure your Theme Location is set in “Appearance > Menus” – switching themes changes the theme location ID, so it’s important to set it (WP tries to remember if the theme has been active before and the theme location has been set, but it’s not perfect).
Same goes for widgets – changing themes can send your widgets to the “Inactive” area, as the widget ID they belonged to no longer exists in the different theme.
Once one theme is deactivated, the styling and functions of are no longer read by WordPress at all, so I’m not sure what styling you’re referring to?
Let me know 🙂