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Choose the menu item Plugins > Manage plugins and click on the button Upload Plugin. Choose the nupkg file that contains the plugin and click on Upload & Install. The plugin has now been installed and can be configured in the plugin management area.
How to install a Theme
Choose the menu item Configuration>Themes and click on the button Upload Theme. Choose the nupkg file that contains the theme and click on Upload & Install. The theme has now been installed and can be configured in the theme management area.
Manual Installation
You can also install extensions manually by uploading it to the corresponding folder. Themes are uploaded into the directory /Themes, and Plugins belong to the directory /Plugins. After the upload, you have to reload the list of Themes by clicking the button Reload Themes in the theme configuration area or for Plugins by clicking the button Edit > Reload List Of Plugins. Now, your new theme or plugin will be displayed in the corresponding configuration area and is ready to be configured and used.