context_system_theme

Provides two additional conditions for the Context module based on using what is identified by Drupal as the current Default Theme and the current Admin Theme.

This varies a bit from what context_condition_theme does in that it doesn't specify a theme ("when using the theme named Bartik, do...") instead you specify which system theme state to apply a context to ("if you're using the system default theme regardless of what the name of that theme is..."). It's more aimed at use-cases of Context that are profile/platform based, not individual site builds (where you'd probably know the name of the actual theme to target using something like context_condition_theme).