Rainglow Color Themes - 320+ color themes for a variety of editors and software.
Froala Design Blocks - 170+ free responsive design blocks based on Bootstrap 4
Slack Themes - Make your Slack pretty with custom themes
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
Antetype - Antetype - Layout aware User Interface Design - responsive and future proof.