Style Switcher

I was thinking about making a gtk4 blueprint template myself, but I think it would be nice to have a style switcher. BlackBox and [My App](https://github.com/halfmexican/mingle/blob/main/src/style_switcher.vala) has one. GJS also as has the pattern with [troll](https://github.com/sonnyp/troll)

![image](https://github.com/user-attachments/assets/73d7a41f-b809-4259-a0f5-16b862b02c6d)
