Add custom theming

highlight.go

@@ -3,11 +3,11 @@ package main
 import (
 	"bytes"
 	"fmt"
-	"path/filepath"
 	"regexp"
 	"strconv"
 	"strings"
 
+	"github.com/alecthomas/chroma/v2/lexers"
 	"github.com/alecthomas/chroma/v2/quick"
 )
 
@@ -252,9 +252,9 @@ func highlightedSource(lexer, source string) string {
 }
 
 func lexerForPath(path string) string {
-	ext := strings.TrimPrefix(filepath.Ext(path), ".")
-	if ext == "" {
-		return "plaintext"
+	lexer := lexers.Match(path)
+	if lexer == nil {
+		return ""
 	}
-	return ext
+	return lexer.Config().Name
 }