Reset build settings to default in Xcode?

Hitting Delete didn't work for me, but that's because I was looking at the settings from the point of view of the target, when the setting was set for the project. Switching to the project view worked -- hitting Delete removed my setting and reverted back to the default (non-bold) value.

When a target's setting is changed Xcode bold it. Also you can find all changed settings in Customized tab. Moreover you can check who exactly influences on the value using Levels tab.

To reset(unbold) a value just select and press delete key