Theme UI is a library created by Brent Jackson for building themeable React apps with Gatsby. It's built with Styled System which provides a constrained set of styling APIs based on props. It uses Emotion and MDX underneath which abstracts us away from a lot of the challenges when dealing with global CSS and styling content.
Theme UI uses "presentational context" to allow a component to apply styling to child elements without introducing global styles. You can use a styles object in your configuration to apply theme-aware styles to ensure consistency.