The default theme hasn't changed from "Default" in ST4, though there have been some changes to it. The default color scheme however has changed from Monokai to

The default theme hasn't changed from "Default" in ST4, though there have been some changes to it. The default color scheme however has changed from Monokai to Mariana. You can easily change it back using `Preferences > Select Color Scheme…`. You can also get back the old tabs - though they do have a reduced feature set - using the setting "file_tab_style".