Fix unknown-terminal-bg color handling

Before this change, we just didn't highlight in this case.

With this change in place, we now fall back on the default dark theme
when background color detection fails.

Fixes <https://github.com/walles/moar/issues/202>.

moar.go

@@ -710,7 +710,7 @@ func pagerFromArgs(
 		return nil, nil, chroma.Style{}, nil, nil
 	}
 
-	var style chroma.Style
+	var style chroma.Style = *styles.Get(defaultDarkTheme)
 	if *styleOption == nil {
 		t0 := time.Now()
 		screen.RequestTerminalBackgroundColor()
@@ -744,6 +744,7 @@ func pagerFromArgs(
 	} else {
 		style = **styleOption
 	}
+	log.Debug("Using style <", style.Name, ">")
 	reader.SetStyleForHighlighting(style)
 
 	pager := m.NewPager(reader)