all: allow combination of default and custom stylesheet

To combine default stylesheet with custom CSS, set the value to "default"
and the file name of custom CSS, separated by comma.
This is the recommended way to use custom stylesheet since it can be set
only on specific pages.

----
// In asciidoctor:
:stylesheet: default, custom.css

// In markdown:
stylesheet: default, custom.css
----

To disable default stylesheet and only using custom CSS, set the value only
to the file name of custom CSS.

Another way to use custom stylesheet is by creating your own template and
pass it to `serve` or `convert` command.
This method makes the stylesheet applied to all pages.

1 files changed, 27 insertions, 13 deletions

diff --git a/filehtml.go b/filehtml.go
index 6e5a080..7731ec0 100644
--- a/filehtml.go
+++ b/filehtml.go
@@ -39,6 +39,10 @@ func (fhtml *fileHTML) unpackAdocMetadata(doc *asciidoctor.Document) {
 		k string
 		v string
 	)
+
+	// By default `:stylesheet:` is set in Attributes.
+	// If user remove it, by using `:stylesheet!:`, the key would not
+	// exist in the map.
 	var withStylesheet bool
 
 	fhtml.Title = doc.Title.String()
@@ -47,10 +51,14 @@ func (fhtml *fileHTML) unpackAdocMetadata(doc *asciidoctor.Document) {
 	for k, v = range doc.Attributes.Entry {
 		switch k {
 		case asciidoctor.DocAttrStylesheet:
-			if len(v) != 0 {
-				fhtml.Styles = append(fhtml.Styles, v)
-			} else {
-				withStylesheet = true
+			vals := strings.Split(v, ",")
+			for _, v = range vals {
+				v = strings.TrimSpace(v)
+				if v == `` || v == `default` {
+					withStylesheet = true
+				} else {
+					fhtml.Styles = append(fhtml.Styles, v)
+				}
 			}
 		case asciidoctor.DocAttrAuthorNames:
 			fhtml.Metadata[asciidoctor.DocAttrAuthor] = v
@@ -86,10 +94,20 @@ func (fhtml *fileHTML) unpackMarkdownMetadata(metadata map[string]any) {
 		key = strings.ToLower(key)
 		switch key {
 		case asciidoctor.DocAttrStylesheet:
-			if vstr == `false` {
-				withStylesheet = false
-			} else {
-				fhtml.Styles = append(fhtml.Styles, vstr)
+			vals := strings.Split(vstr, ",")
+			for _, v := range vals {
+				v = strings.TrimSpace(v)
+
+				switch v {
+				case ``:
+					// Skip it.
+				case `false`:
+					withStylesheet = false
+				case `default`:
+					// Use embedded stylesheet.
+				default:
+					fhtml.Styles = append(fhtml.Styles, vstr)
+				}
 			}
 		case metadataTitle:
 			fhtml.Title = vstr
@@ -105,12 +123,8 @@ func (fhtml *fileHTML) unpackMarkdownMetadata(metadata map[string]any) {
 func (fhtml *fileHTML) initCSS(withStylesheet bool) {
 	var logp = `initCSS`
 
-	if len(fhtml.Styles) != 0 {
-		// User defined their custom CSS throught ":stylesheet:"
-		return
-	}
 	if !withStylesheet {
-		// User unset the stylesheet throught ":stylesheet!".
+		// User explicitly unset the default stylesheet.
 		return
 	}
 	css, err := staticfs.Get(`/ciigo.css`)