Installation

Install through the Craft store or through composer composer require ryssbowh/craft-themes:^4.0
Install plugin in control panel
Install/Create a theme
Add a rule in the settings to load a theme or set a default theme.
If you're using the Pro version, your Sections/Category group templates should extend or look like themed_page and render your theme's regions

Be aware of the following rules regarding to installing/uninstalling themes :

Uninstalling this plugin will uninstall all themes.
Disabling this plugin will disable all themes.
Installing a theme that extends another will automatically install that other theme.
Disabling a theme will disable all theme that extend from it.
Uninstalling a theme will permanently delete all its blocks/displays/layouts (Pro edition only)
Switching from pro edition to lite edition will permanently delete all blocks/displays/layouts