This is an alteration on the Material Palenight theme, that brightens up some of the colours to increase readability.
The original theme that this was based on can be found here.
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Using IDE built-in plugin system:
Preferences > Plugins > Marketplace > Search for "palenight-intellij-theme" > Install Plugin
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Manually:
Download the latest release and install it manually using Preferences > Plugins > ⚙️ > Install plugin from disk...
Plugin based on the IntelliJ Platform Plugin Template.