Add a Theme Editor

[cyacedev]

description

Add a Menu to manage all themes, cloning the default ones and editing them.

acceptance criterias

• [ ] Seperate Themes from Settings
• [ ] Add Buttons in User-Menu
• [ ] Make Default Themes uneditable
• [ ] Implement Cloning, Editing and Deletion of Custom Themes
• [ ] Preview Changes in Demo Window
• [ ] Opt. Share Custom Theme