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
Themer - Personalize themes for your terminal/editor/wallpaper/slack
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Lip Gloss - Design terminal user interfaces
Antetype - Antetype - Layout aware User Interface Design - responsive and future proof.