They are in the project that you can find in the "demos" folder of your local installation. The path matches the namespace, as with components by default. I am also attaching a short video that shows how you can see the relevant source code. If there is demo-specific CSS it is in inline <style> tags in the demo component. The Overview demo of the TileLayout does not have styles, and the shared styles used by the entire app are referenced from its wwwroot folder.
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