Summary: Bug fix. Currenty Flipper provides incorrect red and green diff colors in Dark theme, this PR fixes the issue. Light theme colors are the same. Closes https://github.com/facebook/flipper/issues/3011 ## Changelog Color theme fix: make diff background colors be defined in theme LESS files Pull Request resolved: https://github.com/facebook/flipper/pull/3065 Test Plan: I used [redux-debugger](https://github.com/jk-gan/flipper-plugin-redux-debugger) plugin to test this as this is what I actually use for debugging Redux in React Native projects with Flipper. Here's a [React Native app](https://github.com/3rdp/flipper-redux-debugger-app) that I put together for testing purposes. Light theme:  Dark theme:  Reviewed By: mweststrate Differential Revision: D32529276 Pulled By: passy fbshipit-source-id: 3d11edaf1112444bb106521b6b30db4a4c2b8202