There is a draw.io extension for VS Code (like another comment mentioned, it looks like the main application is vs code), and the UI elements inside the open tab with grid looks like draw.io’s minimalistic dark theme.

Edit: yeah looks like it: https://marketplace.visualstudio.com/items?itemName=hediet.vscode-drawio

The screenshot you have in your post just has a different drawio theme than the default one. You can change the theme in settings of drawio.

Edit 2: Scrap that, it was proved to be obsidian based on LordeMostarda’s comment and answers below