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/themes/font.css

@@ -0,0 +1,4 @@
+@import url("{{.FontURL}}");
+body {
+  font-family: "{{.FontFamily}}", "arial", "roboto" !important;
+}