This is only short description, the complete will come in the full release.

Breaking Change! Changed SCSS filename case

• build/scss/AdminLTE-raw.scss → build/scss/_adminlte.raw.scss
• build/scss/AdminLTE.scss → build/scss/adminlte.scss
• build/scss/AdminLTE-components.scss → build/scss/parts/adminlte.components.scss
• build/scss/AdminLTE-core.scss → build/scss/parts/adminlte.core.scss
• build/scss/AdminLTE-extra-components.scss → build/scss/parts/adminlte.extra-components.scss
• build/scss/AdminLTE-pages.scss → build/scss/parts/adminlte.pages.scss
• build/scss/AdminLTE-plugins.scss → build/scss/parts/adminlte.plugins.scss

New in v3.1:

• Dark Mode
• New Components
  • Month Pagination
  • Login/Register v2
  • IFrame mode
  • uPlot Charts
  • Kanban Board
  • Pages
    • FAQ
    • Contact Us
  • Search
    • Simple
    • Enhanced
  • Simple Kanban Board
• New Demo Examples
  • Stepper/Wizard
  • Multi File Upload
  • Codemirror
  • Datatables with Buttons
  • Form Input Styles
• New Sidebar features
  • Sidebar Search
  • Custom Area
• New JS Plugins & JS Plugin features
  • Fullscreen mode
  • SidebarSearch plugin
  • ExpandableTables plugin
  • IFrame plugin
• Additionally
  • added FontAwesome light, duotone & svg support
  • added Datatables Scrollpane Plugin
  • added btn-app Color Variations
  • enhance brand-link with pushmenu inside brand
  • added new Border Bottom only input style
  • added custom-control-input Color Variations
  • added custom-control-input-outline for checkbox & radio

For the complete changelog look here.