Rainglow Color Themes - 320+ color themes for a variety of editors and software.
Bootstrap Themes - Official themes from Bootstrap's creators
Slack Themes - Make your Slack pretty with custom themes
Ready UI - 200+ Bootstrap UI blocks to create beautiful websites
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.
Bootswatch - Free themes for Bootstrap