Below are simple styles to "flatten" bootstrap. I didn't go through every control and widget that bootstrap offers, only what was required for https://commando.io, so your milage may vary.
This is a CSS snippet that strips Bootstrap's default visual styling to create a flatter, minimalist design aesthetic. It removes shadows, rounded corners, and gradient backgrounds from Bootstrap's core UI components (buttons, forms, alerts, modals, etc.) by applying !important overrides across multiple prefixed properties. The author created this specifically for Commando.io and notes it only covers the Bootstrap elements actually used in that project, so it may need customization for other applications.