Synthesise meta style-entries on demand.

This includes line highlighting, numbers, etc.

Fixes #211.

style.go

@@ -247,15 +247,7 @@ func (s *Style) Builder() *StyleBuilder {
 //
 // This is distinct from Get() which will merge parent tokens.
 func (s *Style) Has(ttype TokenType) bool {
-	return !s.get(ttype).IsZero()
-}
-
-func (s *Style) get(ttype TokenType) StyleEntry {
-	out := s.entries[ttype]
-	if out.IsZero() && s.parent != nil {
-		return s.parent.get(ttype)
-	}
-	return out
+	return !s.get(ttype).IsZero() || s.synthesisable(ttype)
 }
 
 // Get a style entry. Will try sub-category or category if an exact match is not found, and
@@ -268,6 +260,38 @@ func (s *Style) Get(ttype TokenType) StyleEntry {
 		s.get(ttype.SubCategory()))
 }
 
+func (s *Style) get(ttype TokenType) StyleEntry {
+	out := s.entries[ttype]
+	if out.IsZero() && s.synthesisable(ttype) {
+		out = s.synthesise(ttype)
+	}
+	if out.IsZero() && s.parent != nil {
+		return s.parent.get(ttype)
+	}
+	return out
+}
+
+func (s *Style) synthesise(ttype TokenType) StyleEntry {
+	bg := s.get(Background)
+	text := StyleEntry{Colour: bg.Colour}
+	text.Colour = text.Colour.BrightenOrDarken(0.5)
+
+	switch ttype {
+	// If we don't have a line highlight colour, make one that is 10% brighter/darker than the background.
+	case LineHighlight:
+		return StyleEntry{Background: bg.Background.BrightenOrDarken(0.1)}
+
+	// If we don't have line numbers, use the text colour but 20% brighter/darker
+	case LineNumbers, LineNumbersTable:
+		return text
+	}
+	return StyleEntry{}
+}
+
+func (s *Style) synthesisable(ttype TokenType) bool {
+	return ttype == LineHighlight || ttype == LineNumbers || ttype == LineNumbersTable
+}
+
 // ParseStyleEntry parses a Pygments style entry.
 func ParseStyleEntry(entry string) (StyleEntry, error) { // nolint: gocyclo
 	out := StyleEntry{}