feat(middleware): add themed middleware with customizable themes and styles

- Introduced a new themed middleware that allows for dynamic theme application.
- Added support for multiple themes: dark, dark-grey, solarized-dark, and custom CSS.
- Included CSS files for each theme and a font CSS template for font customization.
- Updated middleware registry to include the new themed middleware.

internal/net/gphttp/middleware/middlewares.go

@@ -25,6 +25,7 @@ var allMiddlewares = map[string]*Middleware{
 	"hidexforwarded": HideXForwarded,
 
 	"modifyhtml": ModifyHTML,
+	"themed":     Themed,
 
 	"errorpage":       CustomErrorPage,
 	"customerrorpage": CustomErrorPage,