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/themed.go

@@ -0,0 +1,113 @@
+package middleware
+
+import (
+	"bytes"
+	"fmt"
+	"net/http"
+	"os"
+	"strings"
+	"text/template"
+
+	_ "embed"
+
+	"github.com/yusing/go-proxy/internal/gperr"
+)
+
+type themed struct {
+	FontURL    string `json:"font_url"`
+	FontFamily string `json:"font_family"`
+	Theme      Theme  `json:"theme"` // predefined themes
+	CSS        string `json:"css"`   // css url or content
+
+	m modifyHTML
+}
+
+var Themed = NewMiddleware[themed]()
+
+type Theme string
+
+const (
+	DarkTheme          Theme = "dark"
+	DarkGreyTheme      Theme = "dark-grey"
+	SolarizedDarkTheme Theme = "solarized-dark"
+)
+
+var (
+	//go:embed themes/dark.css
+	darkModeCSS string
+	//go:embed themes/dark-grey.css
+	darkGreyModeCSS string
+	//go:embed themes/solarized-dark.css
+	solarizedDarkModeCSS string
+	//go:embed themes/font.css
+	fontCSS string
+)
+
+var fontCSSTemplate = template.Must(template.New("fontCSS").Parse(fontCSS))
+
+const overAllocate = 256
+
+func (m *themed) before(w http.ResponseWriter, req *http.Request) bool {
+	return m.m.before(w, req)
+}
+
+func (m *themed) modifyResponse(resp *http.Response) error {
+	return m.m.modifyResponse(resp)
+}
+
+func (m *themed) finalize() error {
+	m.m.Target = "body"
+	if m.FontURL != "" && m.FontFamily != "" {
+		buf := bytes.NewBuffer(bytePool.GetSized(len(fontCSS) + overAllocate))
+		buf.WriteString(`<style type="text/css">`)
+		defer bytePool.Put(buf.Bytes())
+		err := fontCSSTemplate.Execute(buf, m)
+		if err != nil {
+			return err
+		}
+		buf.WriteString("</style>")
+		m.m.HTML += buf.String()
+	}
+	if m.CSS != "" && m.Theme != "" {
+		return gperr.New("css and theme are mutually exclusive")
+	}
+	// credit: https://hackcss.egoist.dev
+	if m.Theme != "" {
+		switch m.Theme {
+		case DarkTheme:
+			m.m.HTML += wrapStyleTag(darkModeCSS)
+		case DarkGreyTheme:
+			m.m.HTML += wrapStyleTag(darkGreyModeCSS)
+		case SolarizedDarkTheme:
+			m.m.HTML += wrapStyleTag(solarizedDarkModeCSS)
+		default:
+			return gperr.New("invalid theme").Subject(string(m.Theme))
+		}
+	}
+	if m.CSS != "" {
+		switch {
+		case strings.HasPrefix(m.CSS, "https://"),
+			strings.HasPrefix(m.CSS, "http://"),
+			strings.HasPrefix(m.CSS, "/"):
+			m.m.HTML += wrapStylesheetLinkTag(m.CSS)
+		case strings.HasPrefix(m.CSS, "file://"):
+			css, err := os.ReadFile(strings.TrimPrefix(m.CSS, "file://"))
+			if err != nil {
+				return err
+			}
+			m.m.HTML += wrapStyleTag(string(css))
+		default:
+			// inline css
+			m.m.HTML += wrapStyleTag(m.CSS)
+		}
+	}
+	return nil
+}
+
+func wrapStyleTag(css string) string {
+	return fmt.Sprintf(`<style type="text/css">%s</style>`, css)
+}
+
+func wrapStylesheetLinkTag(href string) string {
+	return fmt.Sprintf(`<link rel="stylesheet" href=%q>`, href)
+}