A preview of available themes is displayed in this dialog. Select a theme by clicking on it.
Use the button to download additional themes from the Internet or load a theme from file.
The button is enabled only for custom themes.
Since Plasma™ 5.6 it is no longer possible to edit and customize individual components or items in a theme.
The recommended tools to check, blend and test themes is the plasma-sdk package, the Plasma Theme Explorer specifically.
If the plasma-sdk package is installed on your system, a button appears hovering a theme preview and lets you start the Plasma Theme Explorer.
For more technical information visit this page.