Creating a custom UI Theme is a process of choosing a base IDE Theme (Light or Darcula) then changing aspects of the base Theme definition. Custom UI Themes can:

substitute icons,
change the colors of icons and UI controls,
alter the borders and insets of UI controls,
provide custom editor schemes,
add background images.

Structure

Theme Basics plugin depends on the IntelliJ Platform SDK and DevKit as a build system.

The main plugin definition file is stored in the plugin.xml file, which is created according to the Plugin Configuration File documentation. It describes definitions of the actions, extensions, or listeners provided by the plugin.

Extension Points

Name	Implementation	Extension Point Class
`com.intellij.themeProvider`	theme_basics.theme.json

Reference: Plugin Extension Points in IntelliJ SDK Docs