Installable themes

[Studio384]

Starting with Luna 2.1, themes will have to be installed, with this a lot of new options are made available to themes.

Backstage

• Rework Themes page to show theme details #1685 Theme selection has been moved to its own page titles "Theme".
• The "Theme" page has been renamed "Appearance".
• Features not supported by a theme are now marked as such.
• Theme can now be removed from the file system from within the Backstage.
• Theme settings are now available if a theme has them.

Themes

• Drop XML in favor of JSON #1677 The information.php file has been replaced with theme.json.
• Define theme features in JSON #1678 Themes can now declare which features they supported in their json declaration.
• Themes can now declare their accent colors in their json declaration.
• Themes can now add configurations to the config-table when installed.
• Theme can now declare the various settings they have in their json declaration.